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3 hours ago, spritzer said:

There are a lot of crap dacs out there but something like the Denafrips and Soekris are excellent.  Made me retire the MSB... 

What about Holo Spring?

On a similar note, i found my used Parasound Z-Dac V1 to be pretty good sounding for the cheapo money i got it for.

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  • 2 months later...

I like it that they are trying to claim this is somehow their design...  seriously?  Old as the hills that one...  Anyway, same as the Egmont etc and not that hard to run with a servo.  Pick a good cathode resistor on the output stage and you are close enough for most servo's to correct.  The balance would concern me

Things I just don't get is why they'd use the 6SL7 and not the ECC83.  ECC83 is superior in every way in something like this where the super high mu is a huge advantage.  Also...why the fuck run it at +/-400V?  They are clearly running with resistors for everything so plenty of voltage being eaten up there but these are ~350V tubes so running them at 800V makes sense?  Bizzarre... 

Also, why have a special transformer for the bias?  Hello...voltage doubler off the B+ limited to +580V with a voltage divider as Stax have done since 1982.  Some other issues as well such as the use of the RN55 resistors which aren't rated to these voltages.  They use them in the HV regulators and well...I'd use CMF60's there.  Cost be damned... 

Seeing this kinda makes me want to try and get an Egmont type amp up and running for this money.  I have a new PCB layout for that one that I can try out.. 

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The SRX uses 12 triodes so that's what they ended up with (or something like it) but this is the SRX:

hdgsta1a.thumb.gif.8e09b8346d0d4fcf02cefb63125b2ffe.gif

Quite an elegant circuit and the cross coupled stuff is something many modern engineers struggle with.  This is what Schiit were doing:

heurcir.jpg.768984a049d51502ffd5e6bf9ba812ed.jpg

So, 6SL7 is just the highest mu (gain) octal dual triode there is so used for the front end to provide gain.  The circuit above uses ECC83's which are more suitable for this role but 6SL7 would work.  Then you have a single cap between the stages and the rest is all DC coupled.  Probably the same as the Triology amp but supposed to sell for 1k$ and not 6k$....  :rolleyes:

xap.thumb.jpg.53b90d51874ef0bc047087ad4d85f389.jpg

Now see how this compares.  6SL7's on the right feeding through a cap (red Wima) to the 6SN7's on the left.  Probably three 20K plate resistors or something like that and the servo on the cathode to control the offset.  Then you have the PSU on the extreme left, two HV supplies with series 350V caps to handle surges (which 500V caps would have handled just fine) and the regulators on the small heatsinks.  Small PSU beneath it to run the opamp, relays and stuff like that.  Bias supply could be the small transistor and diodes to half wave rectify and regulate.  Heaters probably AC as they float at -400VDC. 

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Agree with spritzer.  Now the original 6SN7 is the octal equivalent of the 6CG7/6FQ7 in the SRX Plus, but with only 4 tubes, they are pretty much limited to the Egmont topology, especially since they had both single-ended and balanced inputs.  In fact, Jason Stoddard says, "It’s not a unique design—you’ll also see stuff like this also from John Broskie,"  Here is Broskie's stat amp.  You will see it is very similar to the Egmont posted above, with a wrinkle - specifically, the resistor/cap combo in the input diff amp tail to do some Aikido style power supply noise cancellation.  If Schiit was running the power supply at +/-400 volts I hope they were using the A/B variant of the 6SN7 tubes as that is over the rated voltage of the 6SN7.

Note that the SRX Plus uses 6SN7GTA/B (max static plate voltage 450 volts, max power dissipation 7.5 watts combined) which increases the voltage and power limits a bit over the 6CG7 (max plate voltage 330 volts, max power dissipation 5.7 watts combined) in the original SRX, or the 5965 (max plate voltage 300 volts, max power dissipation 4.0 watts combined)  in Broskie's design.  Using the A/B variant of the 6SN7 in combination with the constant current sources gets the circuit off its knees in terms of drive capability, although it is not nearly as capable as the top Gilmore designs in ultimate drive capacity.  Incidentally, if anyone is concerned with running 6SN7GTA/B tubes at 320-350 volts static plate voltage, RCA has plate curves running up to 650 volts plate voltage.  400 volts on the other hand may be pushing things a bit.

TubeCAD stat amp ~ Egmont.gif

Edited by JimL
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there is only one coupling cap per channel, so the circuit may be slightly different than shown.

plate resistors instead of cascade current source is pretty lame these days.

Completely open loop with no feedback is the result of not enough overall gain, and this is going to make the thing highly unstable.

at least they used Justin's stax jack. The only piece of quality in that mess

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I'm sure there are 4 coupling caps but this angle makes it hard to make out.  Look at the rear 6SN7 and you see just a tiny bit of the other MKP10 cap behind it. 

I'm also sure they are using normal 6SN7's unless the current Russian ones are all A/B types? 

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maybe there are 2 other caps hiding in there. pretty much have to be.

max gain for a 6sn7 is 20 and max gain for a 6sl7 is 70, so 1400 would be max voltage gain at 6% distortion. cut it down to a gain of 500 and you are still at 2% distortion. with no feedback frequency response is going to be much worse at 20khz, like 6db. the resistor current sources make it worse.

stupidly poor design that seems typical of schiit.

kind of wonder that if they ever did make something really nice, people would notice.

 

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  • 2 weeks later...

What started as a spoof and thumbing our noses at Schiit (not literally) and the T8000 is starting to turn into a real amp! Thanks to Kevin for helping me not be an idiot, this time around at least. Gordian knots be damned!

4b2884d258.png

Edited by mypasswordis
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