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Something New in the Works
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Author:  gofar99 [ 02 Sep 2018, 21:27 ]
Post subject:  Re: Something New in the Works

Hi, :thumbsup: I guess close enough. I thought the 12VAC trannie would be a good clue. Push-pull 6AS7 (not P-P parallel though) class A with 6SL7 SRPP driver and digital input and volume control. No phono preamp at this time, but one could be added easily. The ground lug is because I have ground lugs on everything that connects to other gear. Mono blocks are the exception. Since this thing connects to sources like my tape head and phono preamps it will need to have one. Some folks say it is optional...you could fool me as in my system they always result in lower hum and noise. It is set up to be driven either by the internal volume/selection controls or an external set like my existing system preamp.

How does it compare....hard to tell since it isn't finished yet. Expectation is that it will do around 10 watts RMS and respond from below 20HZ to way past 20K. Distortion and S/N are probably in the Oddwatt range of 0.5% at 1/2 power and just at 2% at full power what ever that might end up being at a S/N at 1 watt of about 90db. The extra octal socket on each side is a carry over from early designs and uses 6NO60 delay relay "tubes". I can use them to switch the B+ going to the filter string directly without helper relays. That is because the B+ is only about 165 at the first filter and about 200ma per channel. Well within the ratings for the "tubes". BTW...they are now rather scarce and few reasonable vendors have them at reasonable prices. I have 6 now and will probably use them in future projects ...the average cost was about $10 each (they were common and only $5 when I first used them). Some folks are selling them for (gads) $140 each. When I run out (they are in my 4 existing mono blocks as well) I'll go to the LM555 timer used in the KT120 amps. It actually works better...but is not nearly so cool.

It is just what I really needed....I have far to many amps now, but it is a interesting diversion into P-P triodes.

BTW...I am not noted for doing things half way...but a 40 pound headphone amp is a stretch even for me :D

Good listening
Bruce

Author:  Suncalc [ 03 Sep 2018, 13:10 ]
Post subject:  Re: Something New in the Works

gofar99 wrote:
The extra octal socket on each side is a carry over from early designs and uses 6NO60 delay relay "tubes". I can use them to switch the B+ going to the filter string directly without helper relays.
Ah... Now the socket layout makes more sense. I based my parallel/push-pull guess on the number of sockets (i.e. two 6as7s and one octal preamp SRPP). But with one octal socket being a 6NOxx power relay, this makes the simple push-pull topology much more obvious.

Can't wait to see the final design.

Author:  gofar99 [ 23 Sep 2018, 16:28 ]
Post subject:  Re: Something New in the Works

Hi Everyone, After initial testing I had to redo the PS. Things are now getting a bit complicated. Simple audio section and now 4 distinct power supplies. (12VAC, 2X 130VDC at 250ma, one with two separate 275 VDC sides) I'll know shortly how good it really is. Was very nice at low power before the changes.

Good listening
Bruce
Attachment:
6AS7 Amp Inside 9-2018B.jpg

Author:  ILoveHiFi [ 23 Sep 2018, 18:40 ]
Post subject:  Re: Something New in the Works

From my expreince with pga2311 they aren't any good at amplifying anything, just starts clipping if you want to to amplify with it. Furthermore low input impedance 10k.
However this looks like a neat and good build along with fancy caps can imgaine getting good sound.

Author:  gofar99 [ 23 Sep 2018, 20:17 ]
Post subject:  Re: Something New in the Works

Hi, On the 2311, I ran into that issue with some other chips about 3-4 years ago. They could only do VCC -aboout 3 volts. That would be fine except they ran on 5 volts! With the 2311, my understanding (and past experience) is they don't actually amplify until you cross zero db. Since all I want is the resistor divider inside it ought to be fine. I don't need gain, I have plenty in the amp and a goodly level of output from my sources. I also built in a direct connection to my existing preamp so I can bypass the 2311. I understand that there is a new chip that replaces the 2311 and it is supposed to be better...I just happened to have the other ones already. The new one doesn't have the built in labels that irritate me...CD, DAC etc, just numbers for the inputs in the new one. I want the amp to sing first then see if the front part needs replacement.

Good listening
Bruce

Author:  gofar99 [ 24 Sep 2018, 16:27 ]
Post subject:  Re: Something New in the Works

Hi, It powers up, no smoke, no fire, no blown fuses. All voltages spot on...so tonight I'll test it and give it a listen. I can't do that during the day as my two large black cats are far too helpful.

Good listening
Bruce

Author:  gofar99 [ 25 Sep 2018, 14:23 ]
Post subject:  Re: Something New in the Works

Hi Everyone. Here is your opportunity to poke at me. This is the heaviest 4 watt per channel amp I have ever run into. Somewhere along the line I either have an error in thinking or some bad components. Here is the data. The voltages are as shown in the schematic. I get really clean sines at up to 4 watts, then they start to flat spot on one end and both ends by 5 watts. When I scope and measure things I am not seeing the usual signal swing on the cathodes. Like nearly nothing. Thus the second triode can't do anything. That would account for about 1/2 the output I expected. The drive from the SRPP is over 35 VRMS at the grid of the driven triode. It ought to cause the cathode voltage swing nearly as much up and down to zero (minus the 2-3 volts minimum on the LM317). OK some math....with only 100volts across the 6AS7s and a transformer with a 20 to one ratio the output should reach about 5 volts. Since it is 4 ohms output that should be about 6 watts per tube. My sense is that the second tube is doing nothing and the first one is running like a single ended triode. The harmonic spectrum tends to confirm this. The losses could be about right. So folks what am I missing?

Good listening
Bruce
Attachment:
6AS7 Amp September 24 2018.jpg

Author:  gofar99 [ 26 Sep 2018, 14:50 ]
Post subject:  Re: Something New in the Works

I discovered the answer. :idea: The 6AS7 can not be used in this type of circuit. The actual stage must have gain. With an amplification factor of only two it doesn't in this circuit. An overall gain of at least unity is needed and preferably more. As shown it has a gain of 0.3 approximately. What happens is for every three volts of drive there is only a change of 1 volt on the cathodes. Thus the second tube does not get proper drive. Cranking it up causes the driven tube to go non-linear while the second one is still not really pulling much of a load. Something new to add to the knowledge base.

Good listening
Bruce

Author:  gofar99 [ 27 Sep 2018, 15:51 ]
Post subject:  Re: Something New in the Works

Hi, Not only must the stage have gain but according to Svetlana the way to use these guys is push pull with about 250 V B+ and a drive of 250 PP (yikes). Not something that most circuits can do.

Good listening
Bruce

Author:  gofar99 [ 27 May 2019, 15:01 ]
Post subject:  Re: Something New in the Works

Well it has been a while on the slow burner...but this is now headed toward a modest headphone amp. Actually a rather large change. SE with 6SL7 drivers. It ought to be ready for the public in about a week or so. It seems the 6AS7 makes a nice cap coupled cathode follower for many phones. Zo calculated is 41 ohms with response from 8HZ to a long way past 20K. BTW the other phone amp (p-p 6DJ8) with a SMPS power supply was in the present issue of Audio Express. They seem to like what I write and the crazy ideas I have.

Good listening
Bruce

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