Audio Mirror 6AS7 PSE 20 wpc mono-blocks
      Copyright 2007 Troels Gravesen

INTRO     SCHEMATICS     RE-BUILD     CAPS

Built by Vladimir Bazelkov at Audio Mirror, Huston, Texas. Link to website: Audio Mirror

Audio Mirror 6AS7 PSE 20 wpc monoblocks

These Audio Mirror 6AS7 PSE mono-blocks have been my true workhorses for more than four years now (2010) and are likely to stay so for years to come. They are reliable and have never caused me trouble. Changing the coupling caps improved transparency considerably and with the mods shown below, these monoblocks are serious contenders to my 300B SET amp and I use them all the time for my DTQWT speakers. In fact, I find these amps so good I have ordered teflon tube sockets and they are regularly on the test bench for further refinements as shown below.
Tweaking these amps over and over again may suggest the initial construction is somewhat flawed, but this is not the case. Making a living from producing specialty products for the hifi community is probably any bit as difficult as producing large horn speakers. There's an enormous interest in exotic gear and getting the very best the industry can offer, but try make a low-wattage SET amp from the very best components and see if you can sell it... We like to drewl over these cost-no-object products, but when it comes to buying it, we usually settle for less. So, these amps come with decent caps, tube sockets, etc., and it's up to us to spend further money in refinement, once the initial bill is paid.

Once and a while we may come across components that just stay put in our system for years and years. Not because they are the very best sounding gear around, but because the overall performance, appearance and not least daily handling makes you feel good. I was seriously thinking of building a PSE from 300B tubes two years ago, but would it really sound significantly better? Probably not much. I also cherish a product where money seems to have been well spent. All transformers here are wound for handling also the 6C33 tubes delivering some 50 watts, thus fairly oversized. The stainless steel chassis doesn't easily get scratched and it's easy to clean and so are the acrylic side panels. Changing speakers and amps is something I do regularly and I need gear that's easy to handle and doesn't take cotton gloves every time I rearrange my set-up. The same goes for my Garrard 401 turntable, which makes me feel good by looks and handling - and not least sound.

I had Vladimir Bazelkov adding 4 ohms speaker taps and a switch for 500 mV and 1000 mV input sensitivity. These mono-blocks run by Transcendent GG line stage at 500 mV input sensitivity made an excellent match, significantly better compared to my previous Audio Research SP16L line stage - until I tried the 5687/7119/6N6P w.o.t. line stage. This line stage, finally finished summer 2009, lifted the sound quality even further and these two components now form the core of my system, supplemented by a Transcendent RIAA + SAC MC trafos.

All tubes for the AM mono-blocks are readily available - and at low cost. I have a range of 6SL7 and 6SN7 tubes but can't hear noticeable differences. I recently bought five 6AS7 out-put tubes from China (eBay - google 6AS7) and they sound as good as the Russian tubes originally supplied. 8 $/ea.... doesn't come much cheaper than this. A seriously oversized output transformer may be part of the good sound and potent bass these amps can deliver. I've had a number of visitors in for comparing my 300B and Audio Mirror amps and they always prefer the Audio Mirrors and I'm sure the AMs win due to the 20 watts compared to the 8 watts from the 300B. There's more punch and drama from the AMs, where the 300Bs deliver that special 300B sound that becomes addictive.

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Mundorf silver/gold/PP coupling caps. These caps are huge - and unnecessary I later found out. 100 nF and 220 nF can do.
Even 40 and 80 nF is enough should you want to try out some Russian silver/micas as shown later.


Left: Black Gate type N for input stage cathode by-pass, 2 x 100 uF/16 V non-polarized.
Right: Black Gate VK by-passing cathode resistors, 22 uF/350 V.

Audio Mirror schematic, amplifier stage
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Audio Mirror 6AS7 PSE amplifier stage. After changing to DC filament supply all the 100 ohms resitors
seen above were removed and 100 oms resitors were connected from filament minus to ground. See below.

The switch at 6SL7 cathode resistor provides either 500 or 1000 mV sensitivity. I removed this and used the switch for shorting the input when needed (convenient when change of preamp, etc.)
The amp comes with 2 uF coupling caps, which is rather excessive. Between 6LS7 and 6SN7 we can get along with 220 nF, providing an F3 of 5 Hz. Between 6SN7 and 6AS7 tubes we can do with 100 nF. I actually tried 40 nF Russian Micas and it works fine, actually I'm not sure these Micas aren't as good as the excessively expensive Mundorf silver/gold/PP caps.
Not having a decoupling cap after 2k2 at 6SL7 must be an error from the constructor. 22uF/350V BG was inserted.

The schematics shown here is without any guarantee and I'll see if I can work out the power supply section, which is bit more tricky as it's partly a PCB set-up. So please do not ask how it looks.

The mains trafo has two taps for filament supply. 12.6 V for the 6AS7 out-put tube coupled in series and 6.3 volts for the input/driver tubes. I made these into DC to reduce noise, which really didn't help much I must say. Not that hum is a problem here, but we can always try to improve things if possible. The 100 ohm resistor from V2/pin8 to ground was removed. All filament supplies' negative was connected to ground via 100 ohms.

Input phono sockets' ground connection were insulated from chassis and power supply's central ground was connected to chassis via 10 ohms resistor.

The HT rails are delayed 90 sec. after turn-on.

The mains trafo also has a 100 volt tab (for negative bias) not used in the 6AS7 version, which points to the fact that the 6C33 (2 x 40 watts) version of this amp uses the same mains transformer and output transformer as this 20 watt version.

I feel certain the output transformer has 600 ohms primary winding. Output taps for 4 and 8 ohms. I actually never use the 4 ohms taps.

Future tweaks:
New teflon sockets for all tubes and complete rewiring of amp.
Kiwame resistors for signal paths.
Move on-off switch to rear of amps (having 230 volt right next to the input stage is not ideal)
Further coupling cap tests, VCaps?, Mundorf silver/gold/oil?, tin/teflon? Hard to tell where it ends. Trying some "no-cost" Russian silver/micas has made me wonder whether I really need to spend 300 USD on super caps...


Power supply schematics - including my mods (DC filament supply).
As can be seen the power supply is hugely overdimensioned, e.g. 230 watts for a 20 watts amplifier!

Caps - the necessary evil
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Before starting the above list of modifications, I took another round of cap tests. Once more the green Russian PIO caps were in and they really are nice and easy to listen to. Overall smooth sound given the price, but somewhat lacking in level of transparency and detail. Tonal balance is good, but knowing there is something better on all parameters just doesn't leave them much chance. Trying the Russian FT3 alu/teflon caps was out of the question. I don't think they're that great and the choice of caps here is a cost-no-object issue, except for some ridiculously expensive Jensen silver caps.
What's needed is 2 x 100 nF and 2 x 220 nF, even smaller values can do. The new SCR tin/teflons are 250 USD for the four caps. VCaps are 278 USD, only a little more than the SCRs. Mundorf Supreme silver/gold/oil is 241 USD (prices from VHAudio (US), HiFiCollective (UK) and Strassacker (DE). Jensen silver would make 675 USD for comparison. I find 250 USD for the very best caps acceptable, considering the cost of the amps.


Left: Russian PIOs. Right: New Angela tin foil/PP made by SCR (Solen).

After the green Russian PIOs, I dug up my Russian silver/micas again. The AM amps can really do with only 40 nF and 80 nF, so I paralleled two 40 caps for the first stage. These silver micas cost nothing; that is e.g. 18 USD for 8 x 40 nF. Now, 18 USD is not exactly even close to 250 USD, but how about the sound? The midrange is very close to the VCaps - and maybe even better. They deliver a scaringly presence quality that initially made me think they had a slightly veiled treble performance, but that isn't so. The depth of the soundstage may seem a little shorter compared to the Mundorfs, but every time we try out something significantly different from what our ears have been used to for months, we have to adjust our attention and question ourselves what it was we were listening to before. Because there is no right here, only compromises.

Having received the Obbligato (SCR) tin-foil/teflon caps I quickly inserted these and even out-of-the-box they immediately made it clear that we here have an exceptionally good product. What they do in comparison to the silver/micas is clearing up the treble range. I never really could decide what I should say about the treble performance of the silver/micas but they do smear treble detail a tiny bit in comparison with the SCRs, that is, when they have to handle complex signals like a full symphonic orchestra. With less complex music, the difference is hard to tell.

I have been using the Mundorf silver/gold/PP in many applications and I like them. The level of transparency is excellent, and except for at slight emphasis on the two top octaves, which in certain applications can make them sound a bit hard, I couldn't ask for more.
The VCaps I only know from my 300B amp and when I use the VCaps I miss the Mundorfs and visa-versa. The VCaps are smooth and detailed and do not have the Mundorf silver/gold/PP treble emphasis. The VCaps are simply more right, I think.

The final choice was the SCR tinfoil/teflons as can be seen below.


Audio Mirror Rebuild
2009
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Cleaning up the amplification stages takes like an hour. Rebuild . . . many late night hours. New sockets and soldering tag strips in place.


Left: SCR tinfoil/teflon coupling caps. Right: Massive output transformer for only 20 watts.


Kiwame resitors for signal, metal film for HT supply. All silver plated copper in teflon wire and cables.

Now: Time for some music. AM mods will be over for this time.
Future: Possibly rebuild power supply section.