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A Croatian high-end OTL headphone amplifier

post #1 of 16
Thread Starter 

Hello!

 

This is a follow-up on an article published in Croatia several years ago about a high-end and yet reasonably priced OTL headphone amplifier.

 

Let me first share a few pictures:

 

[IMG]http://i43.tinypic.com/2v12omr.jpg[/IMG]

[IMG]http://i39.tinypic.com/2ym6tjp.jpg[/IMG]

 

The amplifier was constructed to meet the following specifications:

 

no output transformers

zero negative feedback

single ECC88 and one 6H30 per channel

dual mono configuration

class A throughout the operating range

has to conform with the mehahertz criteria

AC tube heating

32 Ohm load capability

short signal path

 

The result was the amplifier shown in the pictures. Only prototype boards were used, there are no printed circuit boards inside. Signal path components were soldered by point-to-point technique by hand, arguably the best way to do it when tubes are in question. The signal path cannot be any shorter than it is so both the output and input connectors are on the front. To satisfy the megahertz criteria, the amplifier has five-times RC filtering per channel and the tube heating is done by AC rather than DC. Despite the slightly higher noise floor, the AC provides for higher linearity. The response extends beyond the 1.2 MHz mark and all high frequency modulation is well beyond the audio range as a result. More or less perfect linearity is achieved between 12 Hz and 80 kHz and slightly less between 8 Hz and 120 kHz at -3 dB roll-off.

 

Components used were mostly European and Russian (old Soviet) componentry acquired in large quantities, measured and selected by hand to ensure minimum tolerance between the channels. Russian paper-in-oil capacitors were used as well as Wima (red) for bypass. I like Holco military grade resistors so that is what was used. The tubes are Russian military specified tubes manufactured in a certain hangar and under a certain serial number. The 6H30 tubes were, of course, matched. 

 

The amplifier was built to allow for usage of 32 Ohm headphones. While it was possible to achieve a sufficiently low output impedance, the amplifier still has more control over headphones with higher impedance. This is then something of a proof that manufacturers of tube amplifiers employ massive amounts of feedback to counter for this problem. As such, those amplifier cannot, and that is a mathematical certainty, be linear. 

 

Cheers!

Antun

post #2 of 16

Very nice work! Any more info the schematic and layout?

post #3 of 16

Very nice.  What kind of headphones have you listened with?

 

As you remark, I had also heard that DC heaters produced less noise. Is there a simple explanation for why the AC heater current provides greater linearity?  I had no idea...

post #4 of 16
Thread Starter 

Hey guys!

 

I am preparing a simplified schematic with both channels included so when that's done, I will post it.

 

About the AC heating, there would seem to be a couple of explanations but I don't have evidential proof as of yet. What I do think it has to do with lower impedance of the AC circuit. However, one has to be very careful with proper twisting of the wires or otherwise you will get hum. There are still some problems to be solved but I am pleased with how the things turned out.

 

During the build, the amplifier was being tested with the Sennheiser HD600 headphones and after the build with the HD650 headphones. I also used the M-Audio IE-40 earphones which are 32 Ohm impedance. The HD650 seemed to have scaled up quite a lot and I don't have an amplifier in my collection that sounds as good with these headphones. If euphony is something you value, then this combination will certainly deliver.

 

I will also post a subject on a DIY triode-based DAC to perfectly complement this amplifier. It's based on a relatively cheap CS4398 board that can be bought on eBay. Though that make come as a negative surprise to some, there's no question that the DAC's performance is largely dependent on the analogue stage and this is where significant improvements can be made. 

 

Cheers!

Antun

post #5 of 16

Hi Antun,

 

The specifications of this amplifier look very good. Can you talk more about how it sounds? Perhaps comparing it with other headphone amps that you've heard. Also, is a completed amp available for purchase?

 

Cheers, 

Andy.

 

Quote:
Originally Posted by R Giskard View Post

Hey guys!

 

I am preparing a simplified schematic with both channels included so when that's done, I will post it.

 

About the AC heating, there would seem to be a couple of explanations but I don't have evidential proof as of yet. What I do think it has to do with lower impedance of the AC circuit. However, one has to be very careful with proper twisting of the wires or otherwise you will get hum. There are still some problems to be solved but I am pleased with how the things turned out.

 

During the build, the amplifier was being tested with the Sennheiser HD600 headphones and after the build with the HD650 headphones. I also used the M-Audio IE-40 earphones which are 32 Ohm impedance. The HD650 seemed to have scaled up quite a lot and I don't have an amplifier in my collection that sounds as good with these headphones. If euphony is something you value, then this combination will certainly deliver.

 

I will also post a subject on a DIY triode-based DAC to perfectly complement this amplifier. It's based on a relatively cheap CS4398 board that can be bought on eBay. Though that make come as a negative surprise to some, there's no question that the DAC's performance is largely dependent on the analogue stage and this is where significant improvements can be made. 

 

Cheers!

Antun

post #6 of 16
Thread Starter 

Hi Andy!

 

Well, this amplifier is not your average tube amplifier so it sounds rather different. Tubes and negative remarks associated with them are not really present. There is no mid-range bloom or lack of control in the bass. This is due to substantial power filtering. Imaging is quite good actually and is the result of painstaking matching of resistor values on both channels as well as tube matching. Generally, it is extremely smooth sounding which is not to say it lacks dynamics. The stage will be heavily dependent on headphones used but Beyerdynamic T1 seems to work perfectly well with this amplifier.

 

I have several amplifiers but unfortunately, I doubt you've heard any of them so comparing this amplifier to those wouldn't be a good reference. However, I will say that Ray Samuels Raptor would be a good point of reference. The Raptor isn't as articulated or as dynamic. It can match this amplifier in terms of mid-range smoothness but not really in the articulation department. The imaging is also quite superior with distinct instrument panning from left to right and back again. The raptor is also quite colored in the bass and lacks control and dynamics. Think of this amplifier as a tube amplifier that doesn't necessarily exhibit all the flaws related with tubes. That's the goal we set ourselves when we started with the design of schematics.  The Raptor is around 1000 $ dollars I think so it was quite pleasing to hear that our amplifier can compete quite favorably. 

 

As for the price/sales, I really haven't thought about that. Everything was built by hand you see and  doubt I'd have the time to make a business out of it. Most likely I'd have to source a company to do it. I also doubt that some would appreciate slightly higher noise floor and superior linearity as a result of AC heating. I doubt there'd be many folks that would appreciate that because it seems to me most people here are like children in a way, throwing in terms like "balanced" or "DC-coupling" and condemning the products that don't offer that just because they have read that those particular features mean the world.

post #7 of 16

Hey Antun,

 

Thanks for your comprehensive reply to my questions. It's interesting that European manufacturers often experiment with different electrical topology, as there are innumerable ways of achieving the same end result and sometimes taking a new approach works wonders. 

 

Perhaps you'd consider offering a bespoke build service as a hobbyist to order? 

 

Cheers,

Andy.

 

Quote:
Originally Posted by R Giskard View Post

Hi Andy!

 

Well, this amplifier is not your average tube amplifier so it sounds rather different. Tubes and negative remarks associated with them are not really present. There is no mid-range bloom or lack of control in the bass. This is due to substantial power filtering. Imaging is quite good actually and is the result of painstaking matching of resistor values on both channels as well as tube matching. Generally, it is extremely smooth sounding which is not to say it lacks dynamics. The stage will be heavily dependent on headphones used but Beyerdynamic T1 seems to work perfectly well with this amplifier.

 

I have several amplifiers but unfortunately, I doubt you've heard any of them so comparing this amplifier to those wouldn't be a good reference. However, I will say that Ray Samuels Raptor would be a good point of reference. The Raptor isn't as articulated or as dynamic. It can match this amplifier in terms of mid-range smoothness but not really in the articulation department. The imaging is also quite superior with distinct instrument panning from left to right and back again. The raptor is also quite colored in the bass and lacks control and dynamics. Think of this amplifier as a tube amplifier that doesn't necessarily exhibit all the flaws related with tubes. That's the goal we set ourselves when we started with the design of schematics.  The Raptor is around 1000 $ dollars I think so it was quite pleasing to hear that our amplifier can compete quite favorably. 

 

As for the price/sales, I really haven't thought about that. Everything was built by hand you see and  doubt I'd have the time to make a business out of it. Most likely I'd have to source a company to do it. I also doubt that some would appreciate slightly higher noise floor and superior linearity as a result of AC heating. I doubt there'd be many folks that would appreciate that because it seems to me most people here are like children in a way, throwing in terms like "balanced" or "DC-coupling" and condemning the products that don't offer that just because they have read that those particular features mean the world.

post #8 of 16

Ah, I've been wondering what you've been up to lately...

 

Maybe this is something that can be offered as a DIY kit?

post #9 of 16
So what does the output stage look like ? I guess a white cathode follower or a srpp stage is out of the way now ... Simple cathode follower with two 6H30 per side ?
edit : the heater-cathode voltage breakdown is pretty high with the 6H30, I may be wrong.
Edited by bidoux - 6/12/13 at 11:13am
post #10 of 16
Thread Starter 

I am preparing a detailed article on the specifics, it should be interesting!

 

I did not think of this as a commercial project. I would love to make this available as a kit but given the current amount of work and as a result, a chronic lack of time, it doesn't seem likely I will be able to do that. I also think this amplifier wouldn't be easy to assemble as a kit because the user would have to be pretty well informed and have knowledge on how to match the components properly. Either that or the components would have to be matched in the factory and nummerized and packed accordingly so the user would at least have one problem less to worry about.  

post #11 of 16

Nice work!

I'm also from Croatia,Zagreb! :)

 

Cheers

post #12 of 16

I've been wondering about the sound and output impendance?

post #13 of 16
Thread Starter 

4 Ohm output impedance with 0 dB negative feedback, hence two 6H30 sections, each with it's own 250VA transformer.

 

The sound depends on RC filter caps but it will be in a very high category. Quality of the tubes will not be critical at all.

 

Upper frequency range extends into MHz range (at around 1.2MHz) but is ruler to 100kHz. Considering it has 5 RC filters, there is no colouration whatsoever.

 

I recommend Mundorf supreme silver/goid/oil or Audio note gold cap for the output. It is absolutely necessary.

 

But I wouldn't recommend building this amp as a DIY project. It is a complex design and the outcome will be highly dependent on part matching as it was built to be a reference, zero compromise amplifier. It is also big, heavy, consumes a lot of power (and I do mean a lot) and runs extremely hot so parts have to be of exceptional quality.

 

There are two switches on the back. One is the heating and the other is the power. The amplifier has to be heated properly before it can give it's maximum but if you do an accident and apply power without heating, nothing will happen. It is designed to withstand a zero Ohm load.

 

The cost of parts would reach somewhere between 500 and 1500 Euro depending mostly on the caps. I considered making a limited series but it wouldn't cost less than 3000 Euro.

 

On a personal note, I'd say it brings the HD650 into world-class category and is even better with LCD-2/3. It won't drive the HE-6. The power bandwidth is perfect so 1st mW sounds as good as the last mW. The response does not alter with change in volume. IEMs are not recommended because even with so much reserves, tubes have their inherent properties that cannot be avoided withoud compromise. In other words, feedback, and that isomething I don't want to do.

 

Cheers!

post #14 of 16

R Giskard; thank you for your answer. I appreciated. 

 

I like OTL amplifiers very much. I am from Slovenia; Europe, and if you agree I'll send you PM. 

post #15 of 16
Thread Starter 

We are neighbors then, I am in Croatia. Sure, send me a PM.

 

But if you're in Slovenia, I think they are quite a few tube designers there.

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