[Ian Bell[_2_]]

I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

[Ian Iveson]

"Ian Bell" wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If you're
willing to use feedback, and the signal is small, you may be
OK. An appropriate filter at the input should make
saturation impossible.

Patrick'll be along in a tick.

Twice the plate load? It *is* the anode load isn't it? 2 to
3 times ra, depending on whether you want power or fidelity
respectively, broadly speaking.

There is a tradition of not caring about head amps...as if
they are add-on extras to the main system. These days, they
quite often *are* the main system.

Ian

[Ian Bell[_2_]]

Ian Iveson wrote:
"Ian Bell" wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If you're
willing to use feedback, and the signal is small, you may be
OK. An appropriate filter at the input should make
saturation impossible.


One thing I was wondering was if I use a transformer with a nominal 16 ohm secondary but use it with
32 ohm headphones for example, then ra reflected into the secondary would be so low relative to the
load that the output could be considered essentially a voltage source. Equally the load reflected
to the plate would be four times higher so distortion would be lower?

Patrick'll be along in a tick.

Twice the plate load? It *is* the anode load isn't it? 2 to
3 times ra, depending on whether you want power or fidelity
respectively, broadly speaking.

Yes, but you are thinking in power amp terms alone and as I have said before I have no power amp
experience. My rule applies to low level transformers e.g. mic transformers and for broadly similar
reasons.


Cheers

Ian

There is a tradition of not caring about head amps...as if
they are add-on extras to the main system. These days, they
quite often *are* the main system.

Ian

[Ian Iveson]

Ian Bell wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If
you're
willing to use feedback, and the signal is small, you may
be
OK. An appropriate filter at the input should make
saturation impossible.

One thing I was wondering was if I use a transformer with
a nominal 16 ohm secondary but use it with 32 ohm
headphones for example, then ra reflected into the
secondary would be so low relative to the load that the
output could be considered essentially a voltage source.
Equally the load reflected to the plate would be four
times higher so distortion would be lower?

Patrick'll be along in a tick.

Twice the plate load? It *is* the anode load isn't it? 2
to
3 times ra, depending on whether you want power or
fidelity
respectively, broadly speaking.

Yes, but you are thinking in power amp terms alone and as
I have said before I have no power amp experience. My rule
applies to low level transformers e.g. mic transformers
and for broadly similar reasons.

10H is in shunt with load. Lower load makes shunt more
significant. Try sim. Should shift 0 to higher F. Use fb to
restore...what then?

Ian, who's English has been wrecked by Twitter.

[Ian Bell[_2_]]

Ian Iveson wrote:
Ian Bell wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If
you're
willing to use feedback, and the signal is small, you may
be
OK. An appropriate filter at the input should make
saturation impossible.

One thing I was wondering was if I use a transformer with
a nominal 16 ohm secondary but use it with 32 ohm
headphones for example, then ra reflected into the
secondary would be so low relative to the load that the
output could be considered essentially a voltage source.
Equally the load reflected to the plate would be four
times higher so distortion would be lower?

Patrick'll be along in a tick.

Twice the plate load? It *is* the anode load isn't it? 2
to
3 times ra, depending on whether you want power or
fidelity
respectively, broadly speaking.

Yes, but you are thinking in power amp terms alone and as
I have said before I have no power amp experience. My rule
applies to low level transformers e.g. mic transformers
and for broadly similar reasons.

10H is in shunt with load. Lower load makes shunt more
significant.

Yeah but, no but, my load is higher not lower.

Cheers

Ian

Try sim. Should shift 0 to higher F. Use fb to
restore...what then?

Ian, who's English has been wrecked by Twitter.

[Ian Iveson]

One thing I was wondering was if I use a transformer
with
a nominal 16 ohm secondary but use it with 32 ohm
headphones for example, then ra reflected into the
secondary would be so low relative to the load that the
output could be considered essentially a voltage source.
Equally the load reflected to the plate would be four
times higher so distortion would be lower?


Language. Lower load = higher resistance.

If you follow my logic, my point is clear. The load
presented by the primary of a transformer comprises:

1. Primary winding resistance
2. Leakage inductance in series with 1
3. Reflected secondary winding resistance in series with 1
and 2
4. Reflected secondary load in series with 1, 2 and 3
5. Primary inductance in shunt with 3 and 4

Think.

Ian

[Ian Iveson]

flipper wrote:

If I understood his meaning he has the relative effect
backwards.

For a given shunt inductance a lower load has better LF
response
(shunt less significant).

This is the first time for many years that the meaning of
the term "load" in this context has been challenged.

Generally, I have avoided the issue and used the less-easily
misunderstood term "load resistance" or just "resistance".
Lately, there has been so little discussion by amateurs that
the jargon of commercial operators, or "professionals" as
they prefer to call themselves, has erected barriers against
incursion. Now the self-styled professionals are getting
sloppy.

An open circuit represents zero load, right? So a high load
resistance is a low load.

I'll try to use more accessible language in the future.

Ian

[Patrick Turner]

On Oct 2, 11:34*am, flipper wrote:
On Fri, 01 Oct 2010 23:51:42 +0100, Ian Bell
wrote:





Ian Iveson wrote:
Ian Bell wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If
you're
willing to use feedback, and the signal is small, you may
be
OK. An appropriate filter at the input should make
saturation impossible.

One thing I was wondering was if I use a transformer with
a nominal 16 ohm secondary but use it with 32 ohm
headphones for example, then ra reflected into the
secondary would be so low relative to the load that *the
output could be considered essentially a voltage source.
Equally the load reflected to the plate would be four
times higher so distortion would be lower?

Patrick'll be along in a tick.

Twice the plate load? It *is* the anode load isn't it? 2
to
3 times ra, depending on whether you want power or
fidelity
respectively, broadly speaking.

Yes, but you are thinking in power amp terms alone and as
I have said before I have no power amp experience. My rule
applies to low level transformers e.g. mic transformers
and for broadly similar reasons.

10H is in shunt with load. Lower load makes shunt more
significant.

Yeah but, no but, my load is higher not lower.

If I understood his meaning he has the relative effect backwards.

For a given shunt inductance a lower load has better LF response
(shunt less significant).

E.g. At 20Hz 10H is roughly 1250 ohms and, relative to the desired
load, 1250 in parallel with 10k (twice the nominal 5k load) is clearly
worse than in parallel with 5k, which is worse than in parallel with
2.5k (half the nominal 5k load).

Thing is, you can't usually accomplish that by simply buying a lower
reflected load transformer because the inductance will be
correspondingly less since the LF frequency response is usually the
same for the same 'quality level' OPT.

What you could do is get a 10k OPT, with likely a 20H primary, and
then, for example, put an 8 ohm load on the 16 ohm secondary, to halve
the load back to 5k, but leakage inductance correspondingly lowers HF
response (the no 'free lunch' axiom).

Unfortunately, with headphone impedances you're working in the other
direction, which aggravates the LF end.

Yes, but the Rsource in parallel with RL and Lp determine the Fco.

if the tube is low Ra, say EL34 in triode then Ra = say 1,300 ohms and
even with no load there at all the Fco is 1,300 / ( 10H x 6.28 ) =
20.7Hz.

An OPT made for a 5k RL may have Lp = 10H and little consideration has
been made for Rsource which might be assumed to be a high Ra pentode.
But also assumed is that NFB will be used.

Now 5k plus 10H gives a load = 3.5k at 80Hz.

So if you had a tube just cabable of driving 5k at 1kHz to say 5W,
then at 80Hz the load has dropped to 3.5k and the output appears
distorted because of the load reduction. It may be possible the OPT is
also saturating.

Suppose you had an OPT meant for 10k, then it'd be likely its LP would
be 20H, and the load will fall to 7k at 80Hz.
The same tube driving this OPT with 10k load will easily produce the
same voltage as for 5W into 5k and the load won't fall to cause
overloading at 80Hz. Such an OPT may be loaded to reflect an anode
load of 5k and the 20H gives a load reduction to 3k5 at 40Hz which is
better.

But loading of the 10k OPT may not reflect low load at all to the
anode and the response with pentode becomes a big arch because gain =
RL x gm.
Triode connection flattens the arch and so does NFB. if the OPT is
rated for 10W to 10k, its Fsat will be for a higher Va than for
a 5W OPT for 5k, so Fsat will be lower with the bigger OPT.

But the winding losses with phones may be negligible, and if P load =
10k for 4,8,16 ohms the the phones might be directly connected to the
4 ohm outlet wih no R divider needed because the OPT ratio 10k:4R =
50:1, and a good enough Vo reduction to lower noise.

I may have said an R divider is a useful thing to reduce noise for
headphones and many amps have such an arrangement. But having low
enough taps on the OPT do the same thing providing that the LL does
not increase so much that HF is cut because such a small portion of
the secondary is used and coupled to the primary. No tube amps I have
seen have a dedicated low Vo tap for headphones. Two resistances are
usually cheaper for the manufacturer.

It is crucial that OPT used for headphone amps and the PT used on the
same chassis be potted lest stray magnetic coupling spoil the outcome.
I recall despairing while trying to reduce hum noise on a MingDa
phones amp. The guys at MingDa also were worried because they tried to
mount flat metal plates between PT and OPT but this crummy fix was
ineffective and what was needed was total re-design of layout and
decent magnetic shielding with pots. GlowOne phone amps had similar
problems. The applied science of making really quiet phones amps with
tubes is still something to be learnt in parts of China.

Patrick Turner.



Cheers

Ian

*Try sim. Should shift 0 to higher F. Use fb to
restore...what then?

Ian, who's English has been wrecked by Twitter.- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

[Patrick Turner]

On Sep 30, 11:47*am, "Ian Iveson"
wrote:
"Ian Bell" wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If you're
willing to use feedback, and the signal is small, you may be
OK. An appropriate filter at the input should make
saturation impossible.

Patrick'll be along in a tick.

That Patrick fella might say "try for as much Lp as possible, and make
sure Fsat at 1kHz clipping level with load of 4x Ra or higher is below
20Hz. "

Sometimes this means you must buy a 10W OPT rated for 10k load and
-1dB at 20Hz - 20kHz.

The use of a genteel little tube like EL84 in triode, or a 6A3 should
give real music with no bad sonics.

Patrick Turner.




Twice the plate load? It *is* the anode load isn't it? 2 to
3 times ra, depending on whether you want power or fidelity
respectively, broadly speaking.

There is a tradition of not caring about head amps...as if
they are add-on extras to the main system. These days, they
quite often *are* the main system.

Ian

[Ian Bell[_2_]]

Patrick Turner wrote:
On Sep 30, 11:47 am, "Ian
wrote:
"Ian Bell" wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If you're
willing to use feedback, and the signal is small, you may be
OK. An appropriate filter at the input should make
saturation impossible.

Patrick'll be along in a tick.

That Patrick fella might say "try for as much Lp as possible, and make
sure Fsat at 1kHz clipping level with load of 4x Ra or higher is below
20Hz. "

Sometimes this means you must buy a 10W OPT rated for 10k load and
-1dB at 20Hz - 20kHz.

The use of a genteel little tube like EL84 in triode, or a 6A3 should
give real music with no bad sonics.

Patrick Turner.



Tha I understand but what intrigued me was your earlier post about how you got perfectly
satisfactory results from a small radio OPT. What sort of Lp would that have had?

Cheers

Ian


Twice the plate load? It *is* the anode load isn't it? 2 to
3 times ra, depending on whether you want power or fidelity
respectively, broadly speaking.

There is a tradition of not caring about head amps...as if
they are add-on extras to the main system. These days, they
quite often *are* the main system.

Ian

[Patrick Turner]

On Oct 1, 8:00*am, Ian Bell wrote:
Patrick Turner wrote:
On Sep 30, 11:47 am, "Ian
wrote:
"Ian Bell" wrote:

It likely depends on how close saturation is to 20Hz, *at
the signal level the OPT is likely to encounter*. If you're
willing to use feedback, and the signal is small, you may be
OK. An appropriate filter at the input should make
saturation impossible.

Patrick'll be along in a tick.

That Patrick fella might say "try for as much Lp as possible, and make
sure Fsat at 1kHz clipping level with load of 4x Ra or higher is below
20Hz. "

Sometimes this means you must buy a 10W OPT rated for 10k load and
-1dB at 20Hz - 20kHz.

The use of a genteel little tube like EL84 in triode, or a 6A3 should
give real music with no bad sonics.

Patrick Turner.

Tha I understand but what intrigued me was your earlier post about how you got perfectly
satisfactory results from a small radio OPT. What sort of Lp would that have had?

The "radio OPT" were from a Trio AM/FM stereo receiver, somewhat a
reasonable quality radio from about 1958, and the OPTs did have
sufficient Lp and low enough LL to give a wide enough BW. The Trio had
SE EL84 in pentode with 6AU6 drivers with 20dB NFB. I removed all the
audio circuits from the receiver to add tubes dedicated towards making
the darn set a pure AM/FM tuner of largely my own design.
The EL84 could get 4W at the OPT sec into 4,8or16R, so 8Vrms was
available to 16R, but this drops if triode mode is used, but it still
worked out OK, and I used global NFB.

Now many mantle radio sets with a single 6V6 output won't have a very
good OPT. The average radio audio circuit with 6V6 is **designed to
make 2W max** from the anode and OPT winding losses with very thin
wire might be 25% so expect 1.5W at clipping and BW 70Hz ( Fsat ) to
10kHz, -3dB. The typical LS has Z = 4 ohms, and OPT ratio might be 7k:
4.

Now 1.5W to 4 ohms = 2.44Vrms, and should phones only need 1V approx,
then Fsat might be at 30Hz.
NFB can straighten out the mess, if one obeys all the rules about
critical damping to achieve stability with low Lp and high LL.

The thing to be remembered is that a mantle radio set produces
terrible sound if turned up loud. It causes domestic bliss to cease,
and rolling pins, plates, and cooking pots to fly across the nation's
kitchens and dining rooms.

I recall an old movie where a TV set which must have had a single 6BM8
as the audio amp was flung out through a window of an appartment on
the 4th floor. Damn thing! Silence! Peace at last!

But at extremly low levels with head against the speaker, the THD/IMD
becomes quite low even with beam tetrode operation and even without a
stitch of NFB.

Patrick Turner.




Cheers

Ian





Twice the plate load? It *is* the anode load isn't it? 2 to
3 times ra, depending on whether you want power or fidelity
respectively, broadly speaking.

There is a tradition of not caring about head amps...as if
they are add-on extras to the main system. These days, they
quite often *are* the main system.

Ian- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -

[Patrick Turner]

On Sep 30, 8:00*am, Ian Bell wrote:
I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

Most old radio OPS designed for a 6V6 operating as a beam tetrode with
no NFB have low Lp because they are designed to give from only 100Hz
upwards as the little speaker used just can't cope with any below
100Hz.

You are right about the 40H. If there is that much L and Ra was 2k2
with EL84 in triode and the RL was a lot higher or not even present,
then response will be -3dB when XLp = Ra or at 8.75Hz, so Lp could be
16H to give -3dB at 22Hz. FB can flatten the resonse further. The real
other problem is that if Lp is low, it is often because the core Afe
size is too small and there are not enough P turns to prevent core
saturation, ie, Bdc + Bac exceeding about 1.6 Tesla at an F that is
too high.
In other words, we want Fsat to be low as possible at the 1kHz
clipping signal voltage.

If one designs for Fsat at below 20Hz then the size of a 5 watt rated
SE OPT can be 4 times heavier than some silly bean counter designed
crap from an old radio. Some such old radios or old hi-fi sets do have
just enough Lp and freedom from saturation at too high an F but they
should be carefully measured before use.
Most old radi OPT just have one sec section wound over the top of a
single primary which gives a poor HF response.

But even if Fsat s at say 50Hz for full Vo, for phone use the Vo may
only be 1/4 full Vo max and Fsat will then be 12.5Hz which s
acceptable.

Patrick Turner.

[Ian Bell[_2_]]

Patrick Turner wrote:
On Sep 30, 8:00 am, Ian wrote:
I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

Most old radio OPS designed for a 6V6 operating as a beam tetrode with
no NFB have low Lp because they are designed to give from only 100Hz
upwards as the little speaker used just can't cope with any below
100Hz.

You are right about the 40H. If there is that much L and Ra was 2k2
with EL84 in triode and the RL was a lot higher or not even present,
then response will be -3dB when XLp = Ra or at 8.75Hz, so Lp could be
16H to give -3dB at 22Hz. FB can flatten the resonse further. The real
other problem is that if Lp is low, it is often because the core Afe
size is too small and there are not enough P turns to prevent core
saturation, ie, Bdc + Bac exceeding about 1.6 Tesla at an F that is
too high.
In other words, we want Fsat to be low as possible at the 1kHz
clipping signal voltage.

If one designs for Fsat at below 20Hz then the size of a 5 watt rated
SE OPT can be 4 times heavier than some silly bean counter designed
crap from an old radio. Some such old radios or old hi-fi sets do have
just enough Lp and freedom from saturation at too high an F but they
should be carefully measured before use.
Most old radi OPT just have one sec section wound over the top of a
single primary which gives a poor HF response.

But even if Fsat s at say 50Hz for full Vo, for phone use the Vo may
only be 1/4 full Vo max and Fsat will then be 12.5Hz which s
acceptable.

Which is an interesting point. Say we have a SET OP transformer with 10H and a 16 ohm secondary.
One watt into 16 ohms need 4V rms whcih is plenty to drive the phones I am intersted in. So suppose
I connect a 32 ohm headphone to this 16 ohm winding and it requires no more than 2V rms (for 125mW)
I would expect lower distortion because of the lower level and higher reflected ac load. What would
happen to the bass response with a 10H primary?

Cheers

Ian

Patrick Turner.

[Patrick Turner]

On Oct 1, 7:58*am, Ian Bell wrote:
Patrick Turner wrote:
On Sep 30, 8:00 am, Ian *wrote:
I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

Most old radio OPS designed for a 6V6 operating as a beam tetrode with
no NFB have low Lp because they are designed to give from only 100Hz
upwards as the little speaker used just can't cope with any below
100Hz.

You are right about the 40H. If there is that much L and Ra was 2k2
with EL84 in triode and the RL was a lot higher or not even present,
then response will be -3dB when XLp = Ra or at 8.75Hz, so Lp could be
16H to give -3dB at 22Hz. FB can flatten the resonse further. The real
other problem is that if Lp is low, it is often because the core Afe
size is too small and there are not enough P turns to prevent core
saturation, ie, Bdc + Bac exceeding about 1.6 Tesla at an F that is
too high.
In other words, we want Fsat to be low as possible at the 1kHz
clipping signal voltage.

If one designs for Fsat at below 20Hz then the size of a 5 watt rated
SE OPT can be 4 times heavier than some silly bean counter designed
crap from an old radio. Some such old radios or old hi-fi sets do have
just enough Lp and freedom from saturation at too high an F but they
should be carefully measured before use.
Most old radi OPT just have one sec section *wound over the top of a
single primary which gives a poor HF response.

But even if Fsat s at say 50Hz for full Vo, for phone use the Vo may
only be 1/4 full Vo max and Fsat will then be 12.5Hz which s
acceptable.

Which is an interesting point. Say we have a SET OP transformer with 10H and a 16 ohm secondary.
One watt into 16 ohms need 4V rms whcih is plenty to drive the phones I am intersted in. So suppose
I connect a 32 ohm headphone to this 16 ohm winding and it requires no more than 2V rms (for 125mW)
I would expect lower distortion because of the lower level and higher reflected ac load. What would
happen to the bass response with a 10H primary?

Its all very well to consider a "SET OP transformer with 10H and a 16
ohm secondary" but you have not mentioned the primary load nor type of
device driving the primary load.

If you have a single EL84 in SE class A1 triode then consider a
typical op point of Ea = +300Vdc and Ia = 30mAdc, and Ra = 2k0 approx..
Total RL + Rw for maximum PO = ( Ea / Ia ) - ( 2 x Ra ) =
( 300/0.03 ) - 4k0 = 6k0 and let us say Rw total is 10% of the totalo
RL so actual RL = 6k0 = 600R = 5k4.
Max PO = RL x ( Idc squared ) / 2 = 2.43W, and anode load Va = 0.707 x
5,400 x 0.03 = 114.5Vrms.
Your OPT has a 5k4 : 16 ohm ratio which gives ZR = 337.5:1 so the Ra
appears at the output as 2k0 / 337.5, plus the total Rw / 337.5 =
5.96R + 1.77 = 7.73R.

This is a not very good outcome unless we add at least about 12dB GNFB
from speaker connection to the driver tube cathode to reduce Rout to
about 2 ohms, giving a DF of 8 with 16 ohm load.

The response at low levels is given as the pole formed by Lp in
parallel to RA, which is Ra + RL in parallel. RDH4 spells it out.
Anyway, assuming the load is resistive at low F where LF cut off
occurs, then RA = 6,000 // 2k0 = 1k5, and 10H has XL = 1k5 at 23.9Hz,
not too bad. If the load around cut off was very high the low triode
Ra manages to keep the source R low so the Fco remains low.

If the sec load is higher than 16 ohms, say 32 ohms or 320 ohms, the
triode Ra remians the main dominant source resistance and Fco will
remain fairly low. My example presumes anode RL with 16 ohm sec =
total of 6k0, and this is 3 x Ra and maybe you get 4% THD at 2 Watts.
With anode RL = high as possible, ie, CCS, then THD will be about 1%
at the maximum Vo but because there is no Ia change there is no PO, so
THD/IMD will vary between about 4% and maybe 2% at maximum *useful*
Vos for RL above 6k0.

But should you use ther EL84 in pentode with Ea = 250V and Ia = 40mA,
Ra = maybe 50k0 and then the Fco is MUCH higher and more dominated by
total RL which may be 0.9 x 250/0.04 = 5k6, if in fact there is a an R
load present at Fco. With 10H and RA = approx 5k0, Fco = 79.6Hz.
The lower the Rsource the lower is the distortions caused by the damn
iron.
EL84 THD in SE mode at near clipping without any NFB anywhere is
usually well over 10%, and fulla odd order H.

Triodes are the gold standard for driving OPTs but one may completely
overcome the shortcomings of pentodes by using NFB in the form of
global, or local CFB or maybe UL plus global.


When I made a pair od SET amps for a customer a couple of years ago I
used 2 x 845 in parallel to get PO max = 60W.
The noise was only barely discernible with head held against a
speaker, and noise was estimated at 0.25mV.This illustrates that one
may use any power tube one wishes to use for either driving a speaker
*or phones*. With a luxury of a 60W output max to 5 ohms, or 17.3Vrms,
one might only want say 3Vrms max at the phones of 32 ohms which is
280mW. The R divider might have R across phones output = 2R2, and feed
R of 10R, so that noise is then reduced to 0.25 x ( 2.2/12.2 )
0.045mV, and if the average phones level was 0.2Vrms, then unweighted
SNR = 20 log 0.000045/0.2 which I guess = -72dB approx and possibly
acceptable, because the background noise of the recording venue is
likely to be higher if nothing else is.

So a single EL34 for phones would be OK, or a 1/2 of 6AS7, and so on,
providing you think it through. PP use of 6SN7 was common with a 20k:
16 OPT.

If one wants to drive down to 8 ohms without an OPT, I'd suggest a
single power mosfet in source follower mode which may be driven by CR
coupling from EL84 in triode. The triode needs a B+ of say +350V and a
CCS feed to anode so that Ea = 250V and Ia = 25mAdc. The EL84 will be
**extremely linear** while producing only up to say 3V into its very
high R loading which is merely the biasing resistors of the mosfet
gates.
Diode clamps would be needed to prevent excessive drive voltage
appearing across Vgate-source. The mosfet would have Ed at +20Vdc and
Idc = 0.5A, and a CCS sink taken to -20vdc, with load connected via
4,700uF.
But better would be to use NPN and PNP mosfets in a complementary pair
and biased for class A1 between the +/- 20Vdc rails.
Direct coupling to a load is then possible if DC offset is controlled.
The Pdd of each mosfet = 10W, and somewhat high to drive phones - 20W
total per channel.
But you can get about 9W of max PO into 18 ohms, 12.7Vrms pure class A
with THD approx 1%. But the noise problem may still be there and a
resistance divider needed. No phones should ever require more than say
1V, so R divider can be 22R + 2R2 is OK and probably class AB action
is OK so Id may be reduced to 250mAdc.
Rout from mosfet followers is about 2 ohms without any NFB, so the
Rout is about determined by the R divider lower resistance.
At 0.2Vrms from the divider, there may be 2V from the mosfets where
THD may be as low as 0.17%. Reducing the THD further means using
global loop NFB and more tube gain. The complementary pair of mosfets
will produce far more undistorted Vo than will one mosfet with
CCS.

Since an EL84 has a gain = approx 20 with CCS loading, then gain with
R divider will be slightly above unity, and about right for use if the
source is a CD player.

One's imagination may easily be employed to make a headphone amp much
simpler and better than the dreadful range of insipid bean counter
inspired designs currently available at excessive prices in hi-fi
stores.

Patrick Turner.



Cheers

Ian





Patrick Turner.- Hide quoted text -

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[Ian Bell[_2_]]

Patrick Turner wrote:
On Oct 1, 7:58 am, Ian wrote:
Patrick Turner wrote:
On Sep 30, 8:00 am, Ian wrote:
I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

Most old radio OPS designed for a 6V6 operating as a beam tetrode with
no NFB have low Lp because they are designed to give from only 100Hz
upwards as the little speaker used just can't cope with any below
100Hz.

You are right about the 40H. If there is that much L and Ra was 2k2
with EL84 in triode and the RL was a lot higher or not even present,
then response will be -3dB when XLp = Ra or at 8.75Hz, so Lp could be
16H to give -3dB at 22Hz. FB can flatten the resonse further. The real
other problem is that if Lp is low, it is often because the core Afe
size is too small and there are not enough P turns to prevent core
saturation, ie, Bdc + Bac exceeding about 1.6 Tesla at an F that is
too high.
In other words, we want Fsat to be low as possible at the 1kHz
clipping signal voltage.

If one designs for Fsat at below 20Hz then the size of a 5 watt rated
SE OPT can be 4 times heavier than some silly bean counter designed
crap from an old radio. Some such old radios or old hi-fi sets do have
just enough Lp and freedom from saturation at too high an F but they
should be carefully measured before use.
Most old radi OPT just have one sec section wound over the top of a
single primary which gives a poor HF response.

But even if Fsat s at say 50Hz for full Vo, for phone use the Vo may
only be 1/4 full Vo max and Fsat will then be 12.5Hz which s
acceptable.

Which is an interesting point. Say we have a SET OP transformer with 10H and a 16 ohm secondary.
One watt into 16 ohms need 4V rms whcih is plenty to drive the phones I am intersted in. So suppose
I connect a 32 ohm headphone to this 16 ohm winding and it requires no more than 2V rms (for 125mW)
I would expect lower distortion because of the lower level and higher reflected ac load. What would
happen to the bass response with a 10H primary?

Its all very well to consider a "SET OP transformer with 10H and a 16
ohm secondary" but you have not mentioned the primary load nor type of
device driving the primary load.

If you have a single EL84 in SE class A1 triode then consider a
typical op point of Ea = +300Vdc and Ia = 30mAdc, and Ra = 2k0 approx.
Total RL + Rw for maximum PO = ( Ea / Ia ) - ( 2 x Ra ) =
( 300/0.03 ) - 4k0 = 6k0 and let us say Rw total is 10% of the totalo
RL so actual RL = 6k0 = 600R = 5k4.
Max PO = RL x ( Idc squared ) / 2 = 2.43W, and anode load Va = 0.707 x
5,400 x 0.03 = 114.5Vrms.
Your OPT has a 5k4 : 16 ohm ratio which gives ZR = 337.5:1 so the Ra
appears at the output as 2k0 / 337.5, plus the total Rw / 337.5 =
5.96R + 1.77 = 7.73R.

This is a not very good outcome unless we add at least about 12dB GNFB
from speaker connection to the driver tube cathode to reduce Rout to
about 2 ohms, giving a DF of 8 with 16 ohm load.

The response at low levels is given as the pole formed by Lp in
parallel to RA, which is Ra + RL in parallel. RDH4 spells it out.
Anyway, assuming the load is resistive at low F where LF cut off
occurs, then RA = 6,000 // 2k0 = 1k5, and 10H has XL = 1k5 at 23.9Hz,
not too bad. If the load around cut off was very high the low triode
Ra manages to keep the source R low so the Fco remains low.

If the sec load is higher than 16 ohms, say 32 ohms or 320 ohms, the
triode Ra remians the main dominant source resistance and Fco will
remain fairly low. My example presumes anode RL with 16 ohm sec =
total of 6k0, and this is 3 x Ra and maybe you get 4% THD at 2 Watts.
With anode RL = high as possible, ie, CCS, then THD will be about 1%
at the maximum Vo but because there is no Ia change there is no PO, so
THD/IMD will vary between about 4% and maybe 2% at maximum *useful*
Vos for RL above 6k0.

But should you use ther EL84 in pentode with Ea = 250V and Ia = 40mA,
Ra = maybe 50k0 and then the Fco is MUCH higher and more dominated by
total RL which may be 0.9 x 250/0.04 = 5k6, if in fact there is a an R
load present at Fco. With 10H and RA = approx 5k0, Fco = 79.6Hz.
The lower the Rsource the lower is the distortions caused by the damn
iron.
EL84 THD in SE mode at near clipping without any NFB anywhere is
usually well over 10%, and fulla odd order H.

Triodes are the gold standard for driving OPTs but one may completely
overcome the shortcomings of pentodes by using NFB in the form of
global, or local CFB or maybe UL plus global.


When I made a pair od SET amps for a customer a couple of years ago I
used 2 x 845 in parallel to get PO max = 60W.
The noise was only barely discernible with head held against a
speaker, and noise was estimated at 0.25mV.This illustrates that one
may use any power tube one wishes to use for either driving a speaker
*or phones*. With a luxury of a 60W output max to 5 ohms, or 17.3Vrms,
one might only want say 3Vrms max at the phones of 32 ohms which is
280mW. The R divider might have R across phones output = 2R2, and feed
R of 10R, so that noise is then reduced to 0.25 x ( 2.2/12.2 )
0.045mV, and if the average phones level was 0.2Vrms, then unweighted
SNR = 20 log 0.000045/0.2 which I guess = -72dB approx and possibly
acceptable, because the background noise of the recording venue is
likely to be higher if nothing else is.

So a single EL34 for phones would be OK, or a 1/2 of 6AS7, and so on,
providing you think it through. PP use of 6SN7 was common with a 20k:
16 OPT.

If one wants to drive down to 8 ohms without an OPT, I'd suggest a
single power mosfet in source follower mode which may be driven by CR
coupling from EL84 in triode. The triode needs a B+ of say +350V and a
CCS feed to anode so that Ea = 250V and Ia = 25mAdc. The EL84 will be
**extremely linear** while producing only up to say 3V into its very
high R loading which is merely the biasing resistors of the mosfet
gates.
Diode clamps would be needed to prevent excessive drive voltage
appearing across Vgate-source. The mosfet would have Ed at +20Vdc and
Idc = 0.5A, and a CCS sink taken to -20vdc, with load connected via
4,700uF.
But better would be to use NPN and PNP mosfets in a complementary pair
and biased for class A1 between the +/- 20Vdc rails.
Direct coupling to a load is then possible if DC offset is controlled.
The Pdd of each mosfet = 10W, and somewhat high to drive phones - 20W
total per channel.
But you can get about 9W of max PO into 18 ohms, 12.7Vrms pure class A
with THD approx 1%. But the noise problem may still be there and a
resistance divider needed. No phones should ever require more than say
1V, so R divider can be 22R + 2R2 is OK and probably class AB action
is OK so Id may be reduced to 250mAdc.
Rout from mosfet followers is about 2 ohms without any NFB, so the
Rout is about determined by the R divider lower resistance.
At 0.2Vrms from the divider, there may be 2V from the mosfets where
THD may be as low as 0.17%. Reducing the THD further means using
global loop NFB and more tube gain. The complementary pair of mosfets
will produce far more undistorted Vo than will one mosfet with
CCS.

Since an EL84 has a gain = approx 20 with CCS loading, then gain with
R divider will be slightly above unity, and about right for use if the
source is a CD player.

One's imagination may easily be employed to make a headphone amp much
simpler and better than the dreadful range of insipid bean counter
inspired designs currently available at excessive prices in hi-fi
stores.

Patrick Turner.



Cheers

Ian





Patrick Turner.- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -


A stunning post hat is just the answer I was looking for Patrick. I have saved that for detailed
digestion along with a cup of tea and RDH4.

Just one small point about damping factor. I am not clear how important this is for driving
headphones. You see many designs with a series output resistor of the same order as the headphone
load they are driving which means a damping factor of 1 or less. Must they therfrtoe sound awful or
is damping factor not sn issue with headphones.

Cheers

Ian

[Patrick Turner]

On Sep 30, 9:58*am, flipper wrote:
On Wed, 29 Sep 2010 23:00:49 +0100, Ian Bell
wrote:

I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

The short answer is yes.

In practice it depends on design and what you mean by 'suffer'.

Open loop the OPT response is what it is. Closed loop is more
complicated because FB will attempt to keep the low end response up;
but saturation is saturation so it can't make full power bandwidth. On
the other hand, 'music' is not a full power monotone and 'average
program level', including bass, is usually at least 8-10dB down even
for compressed 'rock'. So if you put in enough NFB and can muster 1W
of bass from an otherwise 10 watt amp it will 'sound' about right.

The other 'suffer' is you use up the FB dBs in compensating for the
output drop so they're not there to reduce distortion. But, then, the
human ear is not terribly sensitive to low frequency distortion.

Well, the LF harmonic distortion is rarely the problem. But the
intermodulation distortion is a big problem if it is excessive and
caused by an OPT saturating and then all higher F will sound muddied
up.

Patrick Turner.



The net result is you can get bass response that 'sounds' surprisingly
better than what 'just specs' would suggest.