Go Back   Home > Forums > >

Headphone Systems Everything to do with Headphones

New Headphone Amplifier Design
New Headphone Amplifier Design
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools
Old 20th September 2021, 06:21 PM   #21
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
Quote:
Originally Posted by cowanaudio View Post
That's a serious headphone amp. Thanks for sharing.
Thanks, William.

My friend had a first listen last night. He will probably reveal himself here at some point. His comment was that the amplifier sounds quiet and clean but slightly dry and etched compared to the DCG3.

The DCG3 has a second-order dominant distortion (FFT courtesy of Salas in the DCG3 thread) and this is into a 1.5K load. My friend is using 26 Ohm Oppo planar headphones so presumably the current demand, and resulting distortion, are higher than shown on the graph.

I've decided to call this amp the HPA1 since it's a headphone amp and my initials are "HP." So, my impression of the difference between the HPA1 and the DCG3 is that it's like the stereotypical difference between tube and transistor sound. In my advancing old age, I don't have much in the way of top-octave hearing, so I may be missing some distortion that others would readily detect. Or it could be a matter of preferring a slightly more or less colored sound.

Being based on the Blameless topology, this amp might sound like one of those amps. People who don't like the Blameless tend to call it dry and uninvolving. It's hard to define a reference, especially with headphones. The point of the exercise is to try different topologies and see how they sound. I could really use an Audio Precision analyzer right now.

In other news, just in the past couple of weeks, more key parts for this design have gone out of stock at the usual suspects. This may push me to move forward with SMD for continued development of the circuit.

I've uploaded my KiCad project files for the amplifier board to Dropbox. Send me a PM if you would like a link.
Attached Images
File Type: png LineFFT.png (14.1 KB, 327 views)

Last edited by hpasternack; 20th September 2021 at 06:27 PM.
  Reply With Quote
Old 20th September 2021, 07:40 PM   #22
DSP_Geek is offline DSP_Geek  Canada
diyAudio Member
 
Join Date: Aug 2003
Location: Santa Cruz, California
Quote:
Originally Posted by hpasternack View Post
Nice boards. I don't recall reading about this design. I'll look it up.

I think Talema must be struggling with COVID because many of their encapsulated transformers are out of stock at Digi-Key. I lucked out and found the same 15-0-15V transformers with RS labeling on the RS Exports website in England. They were actually cheaper than Digi-Key and arrived in the same amount of time. But the other day I got an email from RS Exports saying they won't do any more business with me until I supply them with a tax exemption certificate.
Not only transformers: Mouser has run out of bloody 2.54 mm Molex connectors, the most common, ordinary things one can imagine. NE5532s? In yer dreams, buddy. Even 115-230 volt slide switches won't show up until January 2022.
  Reply With Quote
Old 20th September 2021, 10:06 PM   #23
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
One hopes that this is a temporary blip and not the beginning of a long-term slide in the supply chain. In general, though, through-hole parts are all going away so if you're not comfortable working with SMD, now is a good time to learn.
  Reply With Quote
Old 25th September 2021, 09:18 PM   #24
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
I've been playing with SPICE and learning more about advanced compensation techniques. Something I failed to appreciate during the design of the amp in this thread is how sensitive the VAS collector-to-base circuit is to tiny capacitances, on the order of 1 pF.

It looks like, from SPICE, that the Baker clamp diode I added has significantly buggered the compensation. It turns two-pole compensation into three-pole compensation, and presumably it's highly-dependent on diode junction capacitance nonlinearity since it's not swamped by a large Cdom. Consequently, I decided to toss the Baker clamp. I added a 1K base current-limiting resistor to the VAS transistor. This doesn't stop saturation, but it keeps the transistor from blowing. SPICE doesn't simulate saturation well, so I won't know how bad the clipping is until I try it in real life.

Adding back-to-back diodes across the LTP cascodes further reduces the amount of VAS base current during overload. This should speed up recovery. The cost is a small increase in distortion.

I also learned this week about putting a Zobel from the input of the diamond buffer to ground to compensate for the diamond's natural RF peaking. I selected 10 Ohms and 390 pF for this.

I was able to retune the TPC with smaller capacitors which reduces the load on the VAS output. The overall effect is a, well, a significant decrease in simulated distortion, more gain margin, higher open-loop GBW, and no loss of phase margin.

My amp is still out for evaluation. The assessment is overall very good, but with a complaint about tonal balance. I have to wonder if all this has anything to do with the subjective performance. I don't have a function generator to do square wave testing, but I plan to get one shortly. When I get the amp back, I will do some more testing, then take it apart and modify it to make these changes. If I'm pleased with the results, I will update the design.
Attached Images
File Type: png FFT.png (26.0 KB, 259 views)
File Type: png BODE.png (54.7 KB, 251 views)
  Reply With Quote
Old 26th September 2021, 02:27 PM   #25
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
Here are a couple of superb papers that derive the equations for two-pole compensation:

https://www.researchgate.net/profile...Amplifiers.pdf
https://www.electronicsworld.co.uk/w...20/04/amps.pdf

The second paper, which is quite long, goes on to compare the Thompson (Blameless) topology to some alternatives, concluding that Thompson "has yet to be superseded in any substantive way." Interesting reading.

The low-distortion-at-all-costs crowd strongly condemn Baker clamps across the VAS and SPICE confirms the negative effect. I don't have a sense whether 0.000004% of added distortion is a problem in real life.

One other issue I've noticed is a weird phase inversion (see attached) on negative peaks that happens when I drive the amplifier strongly at RF frequencies. It should be possible to protect the amplifier from slewing or overload at any frequency when the input amplitude is limited to normal levels. I may have just overloaded the input with my generator. This misbehavior should not be a problem in real-world use, but I need to investigate more. SPICE does not show this effect.

I've been working on a new design based on Cordell's error-correcting MOSFET amplifier paper (http://www.cordellaudio.com/papers/MOSFET_Power_Amp.pdf). The VAS in this design is naturally current-limited. The driver uses clamps but they are referenced to a fixed voltage so don't suffer from Miller-bootstrapping the junction capacitance.

The other day, I ordered some KSC2690/KSA1220 pairs from eBay. These are TO-126 devices, but three times faster than the D44/45H11 devices I use as outputs. Their power dissipation is only 20W, but they ought to suffice with adequate heat sinking and/or moderate bias current reduction. Having faster output transistors makes it easier to hit phase/gain margin targets without excessive compensation.

I'd like to swap the TTA/TTC004Bs for KSA1381/KSC3503s. Alas, while the KSA1381E is readily available, KSC3503s are only sold in D grade and the matching higher-spec parts are gone forever from any reputable supplier I can find.

It might be possible to do away with the TTA/TTCs entirely and use TO-92s here. For bias tracking, you'd like the diamond drivers to be thermally coupled to the outputs somehow.

My friend who is evaluating the amplifier is a well-known diyaudio.com member. There is probably no need to hide his identity, and he has promised to post here eventually, but I prefer not to name him until he's ready to come out of the woodwork.

Emailing with him yesterday, his main observation is dryness in the midrange compared to the DCG3. He's still trying to figure out whether this is a fault of the amplifier, a synergy thing with his low-impedance planar headphones, a matter of equalization, or just a personal preference. He is a single-ended triode guy which says something about his taste. Personally, I like the way the HPA1 (now my official name for the project) cleans up the midrange compared to the DCG3.

One thing I want to emphasize is that this is still a prototype. I believe the amplifier works well as shown, but caution anyone who wants to duplicate it that the design hasn't been fully tested nor tuned for sound quality in any way.
Attached Images
File Type: jpg Overload.jpg (576.8 KB, 230 views)
  Reply With Quote
Old 26th September 2021, 02:36 PM   #26
jackinnj is offline jackinnj  United States
diyAudio Member
 
jackinnj's Avatar
 
Join Date: Apr 2002
Location: Llanddewi Brefi, NJ
New Headphone Amplifier Design
Very nice!
__________________
"All models are wrong, but some are useful" G. Hill
  Reply With Quote
Old 26th September 2021, 02:49 PM   #27
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
Quote:
Originally Posted by jackinnj View Post
Very nice!
Thanks.

I forgot to mention, the frequency in the photo above is 5 MHz.

Also, the output inductors are 14 turns of magnet wire wound on a 3/8" drill bit. After heat shrinking the coils, I smeared some epoxy inside to hold things together. Unfortunately, the spools in my junk box are unlabeled, but the wire diameter looks to be about 0.7mm. The value isn't critical. I measured mine around 1.2 uH or so, IIRC.
Attached Images
File Type: jpeg IMG_4354.jpeg (418.2 KB, 236 views)
  Reply With Quote
Old 26th September 2021, 04:02 PM   #28
DontHertzMe is offline DontHertzMe  South Africa
No Morse
diyAudio Member
 
DontHertzMe's Avatar
 
Join Date: Jul 2013
New Headphone Amplifier Design
Thank you for the ongoing useful information. I look forward to future developments
  Reply With Quote
Old 26th September 2021, 05:05 PM   #29
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
Quote:
Originally Posted by DontHertzMe View Post
Thank you for the ongoing useful information. I look forward to future developments
Again, thanks for the positive feedback. I worry that many of these details could be more information than people want to read. On the other hand, I know people have different levels of technical background/interest, and maybe enough are interested that it's worth the effort.

A lot of this is very subtle and I spend hours reading and thinking, trying to develop my technical intuition. I believe there's a need for articles and discussion that bridge the gap between generalities (like, "Feedback through Cdom linearizes the VAS") and pages of math. I feel I would succeed if I could turn a complicated mathematical idea into something more accessible to math-challenged readers (like myself). But you can only make complicated things so simple.

Later, I may try to write down what I've learned about the theory and advantages of two-pole compensation. I may earn a fortune with a novel insomnia cure.

a child could do it short - YouTube
  Reply With Quote
Old 4th October 2021, 08:48 PM   #30
hpasternack is offline hpasternack
diyAudio Member
 
Join Date: Sep 2002
Location: New England
I said I would write some notes on two-pole compensation, and I've tried a couple of times. But I keep getting off onto long-winded essays that seem inappropriate, so I will just say a few brief things and if anyone wants to discuss it, I will be happy to say more.

I didn't understand TPC until recently and was pleasantly surprised to find it's not as scary as I expected. I was even more surprised how well it worked out in HPA1. The big advantages seem to outweigh the small disadvantages, and now I intend to use TPC (or some other advanced compensation) in my designs going forward.

I posted above some good papers on TPC, but here is a very short summary. Compensation is a race to roll off the loop gain below unity before the phase shift hits 180 degrees (120-135 degrees in practice). With TPC, we start the rolloff faster and later, then back off at the end so we cross the finish line with an adequate phase margin. The downside is the loop phase dips close to -180 degrees at midband. But Nyquist tells us this is ok, provided the phase picks back again near the unity gain point.

In principle, the reduced phase margin at middle frequencies isn't a problem, but clipping or large-signal parameter changes can make the amplifier conditionally unstable. TPC amplifiers seem to behave worse when clipping, and I saw that with my design.

The benefit is much more area between the open- and closed-loop gain curves, which translates to more feedback available for error correction. So, all other things being equal, the amplifier with TPC can have much lower distortion across the band, instead of just at low frequencies as is the case with dominant pole compensation.

There is another benefit, and it is huge. The dominant pole capacitor looks like a short circuit to the input stage at most frequencies. This is because its reactance is negative-bootstrapped (Miller effect) by the voltage gain of the second stage. With TPC, we reduce the amount of feedback around the VAS. Less shunt feedback means the VAS input impedance is higher, and much less current is demanded from the input stage. This has a whole slew (pun intended maybe?) of advantages including reduced IPS distortion.

The VAS doesn't have to swing that much current compared to the output stage, and can be designed for adequate linearity. The total amount of gain in the system remains fixed, but by throwing away less gain with local feedback around the VAS, we can wrap the output stage in more feedback, to good effect overall.

There are two capacitors and one resistor in the TPC network, and the values have to be selected. A formula that seems to work is as follows:

1) Start by making the value of the capacitor connected to the VAS output the same as you would use for dominant-pole compensation.
2) Make the second capacitor ten times this value.
3) Using SPICE, adjust the value of the resistor for a good compromise between phase margin, gain margin, and available negative feedback at midband.

The loop gain will show some peaking at the lower corner frequency. Playing in SPICE, you can get an idea of the effect of varying the values of each component. You can get rid of the peak by implementing three-pole compensation. You do this by adding a third capacitor in series with the resistor. 150nF is a good starting point. Then you spend some time iterating in SPICE and hope your models are accurate. I haven't tried three-pole compensation yet, but plan to in my next design.

As I said earlier, a very small capacitor (1-2 pF) across the whole network completely alters the response. This is in the range of the capacitance of a switching diode, so a Baker clamp from collector to base will mess things up considerably.

You an still get a decent response with the clamp diode in place (New Headphone Amplifier Design) but it doesn't look like the curves you'll see in the textbook examples. This is where I'm at right now with HPA1 and it does seem to work. HPA2 will have a different clamping scheme to avoid this problem.

Feedback theory can get pretty complicated pretty fast, to the point where it makes my brain hurt and I would rather look at Netflix than do math. Buried in this (200W MOSFET CFA amp) thread are some highly technical discussions that seem very interesting but I haven't had the mental fortitude to digest the information yet.

Anyway, that seems like more than enough for a "brief" posting.

As always, FWIW and YMMV.
  Reply With Quote

Reply


New Headphone Amplifier DesignHide this!Advertise here!
Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
FS: The Wire headphone SE-SE headphone amplifier ulogin Swap Meet 67 11th September 2018 04:48 PM
FS: The Wire headphone SE-SE headphone amplifier ulogin Swap Meet 23 30th January 2018 09:01 PM
Headphone Impedance and headphone amplifier TheGimp Tubes / Valves 89 3rd May 2014 01:46 PM
Looking to hire someone to help with simple headphone amplifier design FasterHorses Chip Amps 4 17th May 2011 07:45 PM
My DIY headphone amplifier design cetoole Headphone Systems 26 9th December 2005 01:39 PM


New To Site? Need Help?

All times are GMT. The time now is 05:53 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 13.64%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright ©1999-2021 diyAudio
Wiki