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Old 27th May 2010, 12:31 AM   #1
ChrisA is offline ChrisA  United States
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Default ST-70 build, ideas and advice

I'm thinking of building a ST-70 type amp. I have a pair of 70's vintage Infinity Quantum 2 speakers I can put in an upstairs bedroom. The room is 20 x 16 feet and I don't need the huge about of volume. So while 35 WPC is on the low side of what Infinity recommended, they were assuming a 70's vintage transistor amp. I should be fine with an ST-70, I think.

My plan is to update the design, use a scratch built aluminium chassis and a beefed up power supply, all the connections on the rear, grounded power cord and so on. But otherwise still a Dynaco ST-70


Here is the question: Which of the common ST-70 upgrade driver broads sounds the best? OK, bad question. What is the difference in sound between the common ST-70 upgrade driver boards?

Are the "clone" A470 OPTs still the best compared to Hammond or Edcor?

Here are two st-70 driver boards I'm looking at
Driver Boards for Dynaco ST70
diytube.com :: View topic - the diytube ST70 driver

Here is a link to the specs on my speakers
Infinity Quantum 2 Speakers
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Old 27th May 2010, 12:40 AM   #2
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Check out Gregg the Geeks ST70, dynamutt, input board. I can't speak for any of the othersm but my ST70 is REALLY good.

As to OPTs, if you can afford them, the O'Netics are very good.

Once you've done everything, the only similarity between an ST70 and the amp you build is that they will be PP EL34 with fixed bias.

dave
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Old 27th May 2010, 01:17 AM   #3
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Triode Electronics' ST70 driver board populated with 2X EF86 and an ECC99 gets my nod. Hafler's topology is retained, while the active devices used are much better.

A pentode I/P stage allows the use of 470 KOhm grid to ground resistors, without worrying about HF losses from CMiller interactions. That, in turn, allows you to use a 50 KOhm passive volume control. As this is for a bedroom system, an "integrated" amp may be your best bet. Somehow, I don't see LPs being a signal source.
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Old 27th May 2010, 04:04 AM   #4
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I use a 12AU7-modified version of the original circuit. I don't need the extra gain because it does not need feedback. I cross-couple the outputs into the cathodes and put a 15 ohm resistor to ground from the 4 ohm tap. So there is feedback at the output (still ultra-linear) but none to the 12AU7 voltage amplifier. Usually with a little cut and splice I can get this to go onto the original board.

The advantage is that there is no loop feedback => sounds better!
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Old 27th May 2010, 04:31 AM   #5
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by atmasphere View Post
...

The advantage is that there is no loop feedback => sounds better!
Why would this be? I understand the theory about how feedback should control distortion but I assume there is a side effect?




What about moderate price OPTs? The A470 clones are basically $100 each I consider $100 plus or minus $25 "moderate".

What I want is a nice secondary system. I was thinking of s low power SET and 100+ dB speakers. But then I hate to keep the pair of Infinity Q2 packed away in storage.
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Old 27th May 2010, 07:29 AM   #6
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Quote:
Originally Posted by atmasphere View Post
I use a 12AU7-modified version of the original circuit. I don't need the extra gain because it does not need feedback. I cross-couple the outputs into the cathodes and put a 15 ohm resistor to ground from the 4 ohm tap. So there is feedback at the output (still ultra-linear) but none to the 12AU7 voltage amplifier. Usually with a little cut and splice I can get this to go onto the original board.

The advantage is that there is no loop feedback => sounds better!
Could you please attach the schematic?

Thanks in advance
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Old 27th May 2010, 08:07 AM   #7
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Here is the link to the DynaMutt board. Gregg also has a board based on the original, and the front end from Kevin Kennedy (see post #1)

dave
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Old 27th May 2010, 09:45 AM   #8
SY is offline SY  United States
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Quote:
Originally Posted by ChrisA View Post


What about moderate price OPTs? The A470 clones are basically $100 each I consider $100 plus or minus $25 "moderate".

What I want is a nice secondary system.
Why not use the original transformers? they really are quite good, especially the earlier ones with cloth-insulated leads.
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Old 27th May 2010, 03:18 PM   #9
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by SY View Post
Why not use the original transformers? they really are quite good, especially the earlier ones with cloth-insulated leads.
That was the question, Are the original transformers still the best ones to use? I assume to consensus is "yes".
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Old 27th May 2010, 03:19 PM   #10
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Best? No. Very good and more than usable? Yes.
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