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Improved Tube Headphone Amp Multi-Ef­fects Plugins
Old 27th June 2011
  #1
Improved Tube Headphone Amp

For many months I have been trying to develop an all tube headphones amp that can drive a wide range of headphones to a high level with low distortion. The designs have been based on a transformer output using a Sowter 8665A to match the tube stage to headphones of different impedances. With nearly 200GBP worth of transformers in the design this was most certainly not a poor man's headphones amp!! I have tried cathode followers, mu followers and White followers, open loop and and with NFB but with none of them could I get much more than 125mW output without a spray of harmonic distortion. So I abandoned the task for a while.

Then, back in May, I came across Pete Millett's SRPP design using the ECC99. I had not tried an SRPP design because of their reputation for high distortion so it was almost in desperation that I knocked up a prototype. Sure enough it had quite high distortion but I noticed two interesting things. First it was quite capable of delivering 3V rms into a 32 ohm load (280mW) which is enough to drive just about any headphone to an almost painfully loud level. Secondly, although it produced 3% THD at this level, the harmonics fell away rapidly, most of the distortion being 2nd and 3rd harmonic. This was in stark contrast to other designs I had tried.

That's as far as Pete Millett's design goes. There is a preceding amplification stage but no NFB so although it can provide a high output, the distortion is rather high. It seemed to me it might benefit from some NFB, so I designed a single triode stage base on one half of a 12AX7 and closed the loop from the SRPP output back the the 12AX7 cathode. Unfortunately, because there is a dc blocking capacitor in the feedback loop the closed loop gain rises at very low frequencies and it is not possible to apply enough NFB to reduce the distortion significantly without instability.

The classic way to ensure unconditional stability in tube NFB circuits is to ensure there is only a single low frequency pole in the loop which as often as not means the NFB network has to operate down to dc, i.e no series caps in the NFB loop. This can often be problematic from the point of view of setting the dc conditions in the tubes and this case was no exception and a compromise had to be made in slightly unbalancing the SRPP stage to achieve it. Despite that, the results are good:

2V rms into 32 ohms (125mW)

2H = 0.18%
3H = 0.032%
4H = 0.006%

Higher harmonics were immeasurable

3V rms into 32 ohms (280 mW)

2H = 0.28%
3H = 0.063%
4H = 0.014%

Other harmonics immeasurably low.

For 3V rms output into 32 ohms an input of 0.46V rms is required.


I built the prototype using a die cast box as a chassis but I am now well on the way with a PCB layout which looks as though it will fit onto a board 3.5 inches by 5 inches (with the transformers external).

Schematic of one channel attached.

Cheers

Ian
Attached Thumbnails
Improved Tube Headphone Amp-ecc99phonescctsmall.jpg  
Old 9th December 2011
  #2
Gear Maniac
 
Rylan's Avatar
 

Nice, thanks for sharing Ian!
Old 9th December 2011
  #3
Quote:
Originally Posted by Rylan View Post
Nice, thanks for sharing Ian!
The PCB is done now and I have had a few made. Here is a pic of the PCB on the bench but with cheap Edcor output transformers rather than the specified Sowters. The Edcors seem to work reasonably well for 32 ohm headphones but lack the taps for other impedances that the Sowter has.

Cheers

Ian
Attached Thumbnails
Improved Tube Headphone Amp-protowithedcor.jpg  
Old 9th December 2011
  #4
Gear Maniac
 
Rylan's Avatar
 

Which are those? The GXSE5-8-5.5K?
Old 9th December 2011
  #5
Quote:
Originally Posted by Rylan View Post
Which are those? The GXSE5-8-5.5K?
They are XSE10-50-8K used to make 32 ohm headphones look like about 5K to the SRPP ECC99 stage. Not ideal as they are gapped so the inductance is much lower than the Sowter.

Cheers

Ian
Old 9th December 2011
  #6
Gear Maniac
 
Rylan's Avatar
 

I see the frequency response drops at 70 hz on that one, isn't that problematic for low end response? I am somewhat inexperienced on transformer selection.
Old 10th December 2011
  #7
Quote:
Originally Posted by Rylan View Post
I see the frequency response drops at 70 hz on that one, isn't that problematic for low end response? I am somewhat inexperienced on transformer selection.
Depends. Edcor specs are in many ways virtually meaningless. The specified frequency response fails to say anything about the conditions under which the test was made. For most transformers intended for single ended operation, the inductance drops at low frequencies and high powers so if this measurement was taken at 10W then the results would be much better at 1W and even better still at the 100mW or so needed to drive headphones.

The low frequency response is determined by the inductance of the primary and the source resistance driving it. Basically the response will be 3dB down when the primary inductance equals the driving source impedance which for the ECC99 SRPP topology is about 400 ohms. For a 3dB point at 20Hz the primary inductance = 2*pi*20*L = 400 so L needs to be at least 3.18 Henries although with a load that heavy the SRPP stage would distort quite heavily. Edcor do not specify the primary inductance but judging from the size of the transformer I would guess it to be in the region of 10 to 20 Henries which is probably barely adequate and certainly not ideal.

I seem to remember reading somewhere that the primary inductance of the Sowter is 120 Henries.

So, to answer your question, it could be problematic but it is impossible to say from the scant information provided by the manufacturer. The guy in the states who has built one using the Edcors says the bass end sound great.

Cheers

Ian
Old 10th December 2011
  #8
Gear Maniac
 
Rylan's Avatar
 

Ah nice, I need to bookmark this post to refer back to later! Thanks!
Old 10th December 2011
  #9
Lives for gear
 
Wavebourn's Avatar
 

I would prefer to take feedback after caps, from the primary.

Otherwise it looks nice. Reminds my mic pre from Mod Alongs thread.

What software do you use for PCB design?
Old 10th December 2011
  #10
Looks like there will be a bit of band limiting there and a fair amount of THD.

Why the determination of using tubes for a linear headphone amp?

I went with a CFA opamp into Harris HA5001 buffers, that's direct coupled, .0008% THD, zero phase shift, -105 db noise and drives 1/2 amp at very low impedance.

It's super clear, super fast (2000v/us slew rate) huge bandwidth (100 mhz) and no percievable noise. What you put in, you get out, no changes, no colors, no suprizes. I built them for about $50.
Old 10th December 2011
  #11
Gear Addict
Quote:
I went with a CFA opamp into Harris HA5001 buffers, that's direct coupled, .0008% THD, zero phase shift, -105 db noise and drives 1/2 amp at very low impedance.
I'm curious about this Jim. A quick search of the Interwebs pulled up nothing about the HA5001...I did see a datasheet for the HA5004.

I don't really have a desire for a tube (or "tooob") headphone amp...But something that is crystal clear and highly accurate sounds like a good time. I'm pretty sure you have a specific model of CFA opamp that you like to use for this as well.
Old 10th December 2011
  #12
Quote:
Originally Posted by Wavebourn View Post
I would prefer to take feedback after caps, from the primary.
You will no doubt have noticed that the NFB in my design extends down to dc and with only a single capacitor in the forward loop it is unconditionally stable. Neither is the case if you take NFB after the cap. You might extend the very low frequency response a little at the expense of increased distortion but IMHO it is not worth it.

Quote:
Otherwise it looks nice. Reminds my mic pre from Mod Alongs thread.
I am not aware of that; can you point me to the thread?

Quote:
What software do you use for PCB design?

FreePCB.

Cheers

Ian
Old 10th December 2011
  #13
Quote:
Originally Posted by Jim Williams View Post
Why the determination of using tubes for a linear headphone amp?
That's what the client asked for.

Cheers

Ian
Old 11th December 2011
  #14
Quote:
Originally Posted by effitall View Post
I'm curious about this Jim. A quick search of the Interwebs pulled up nothing about the HA5001...I did see a datasheet for the HA5004.

I don't really have a desire for a tube (or "tooob") headphone amp...But something that is crystal clear and highly accurate sounds like a good time. I'm pretty sure you have a specific model of CFA opamp that you like to use for this as well.
Digi key sells them, they are SOIC or dip. They do have a weird pinout. Many top end headphone manufacturers use these output buffers. I was using the BurrBrown Buf634, but Brown Dog came out with these 634 to 5001 converter pcb's that allow you to use 2 buffers in the place of one Buf634. They do sound more pure than the BB part although they spec similar. The opamp is the LME49713 CFA. I'm done with headphone amps, these are all I will ever need.

For those still chasing the vacuum dream, there is a nice tube headphone amp design in Audio Express magazine from a couple years back.
Old 12th December 2011
  #15
Gear Addict
Interesting...I was looking for the wrong stuff. Harris apparently sold the semi biz to Intersil. Now the HA5001/2 chips are actually Intersil parts.

A quick search gave me this as a headphone amp based off of the HA-5002.

PPA v1 Amplifier Board

I'd not bother with the bass boost...But found the driven ground and CCS interesting.

Thoughts?
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