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Old 1st November 2010, 08:35 PM   #1
FrankW is offline FrankW  United States
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Default Driver for Dynaco ST-150

Hi - I am dusting off an old project and am interested in ideas.
I have a lot of time invested in a Dyna ST-150 and know there are much better project amps out there. That said, I still want to finish this one, even though it doesn't make sense to anyone but me.
My ST-150 has good output transistors and a heavily modified power supply - additional 10,000 uFd caps for each rail, in-rush current limiting resistors that drop out via a relay 30 seconds after turn-on, PS caps bypassed with poly caps, IEC connector, Cardas output jacks, etc. - I'm pretty much commited ( or should be!) at this point.
The driver board is toast and even though I like the Boak mods, 1st I'd have to get a good driver board and then modify it.
Could someone suggest an alternate solid state driver board or schematic, maybe something more up to date?
Thanks!
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Old 2nd November 2010, 12:20 AM   #2
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Well, my ST120 has been on fire twice, and my PCB lands didn't go. I'm using NTE 49 and 50 TO220 driver transistors instead of the RCA 5 digit ones my rig came with, and with NTE181 output transistors I like the sound. I think if I did it again I would try TIP41 and compliment or something. I've got little heatsinks on them. With the extra gain of and speed of the 181's and 49's, I think my slew rate is way up and the alleged dull sound is a thing of the past. Somebody did a simulations of a british 70 amp, and changing the output transistors from 2n3772 to 2N3055 to TIP 3055 improved distortion every time. I'm using the DJOFFE St120 bias board (homebuilt) to stabilize output transistors at 22 ma steady state current, which eliminates the crossover distortion that would obtain with the original circuit at low volume with the bias making capacitor not pumped up. Don't know your circuit, post it or a link if you want more knowledgable people than me to comment. I like the ST120 output capacitor, no matter now many slams it gets, it was not the problem. And after two fires, no speakers damaged by DC overcurrent. I put two 12 VDC fans on the output transistors last weekend so I can leave it on over 8 hours unattended, look like little earmuffs around the perforated cover.
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Old 2nd November 2010, 01:11 AM   #3
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Ok, akdatabase.org had the ST150 schematic. Totally different ballgame, forget everything I said except about faster output transistors making for less distortion. Split supplies, one little mistake and your speakers are junk. Needs a diac triac protection circuit anyway to blow the fuses if your output gets too close to the supply rail (shorted output transistor). Can use a dual back to back zener instead of a diac. In split supply one output transistor amps, DJoffe likes the NAIM schematic (UK premium brand) he posted on the thread Dynaco Stereo 120...can be beautiful on this forum. Simpler, allegedly superb. Don't know where you would buy a PCB.
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Last edited by indianajo; 2nd November 2010 at 01:24 AM.
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Old 2nd November 2010, 06:58 AM   #4
djk is offline djk
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"Can use a dual back to back zener instead of a diac."

They don't work the same way.

Actually, you want an SBS. It is a type of low voltage Diac with a synchronizing electrode. We don't need the electrode for this use, but we need the lower 8V~10V trip voltage.

A small board will an SBS (2N4991/92) and a big triac is available from Peavey for a small sum, it mounts on dual banana posts.
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Old 2nd November 2010, 07:14 AM   #5
djk is offline djk
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If you wanted to make this a real project amplifier, convert the output stage to class G.

The transformer already has the extra windings on it. The amplifier would put out 150W/8R when you were done.
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Old 2nd November 2010, 11:41 PM   #6
FrankW is offline FrankW  United States
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I did not expect so may varied replies so soon - thanks!

What I am tryingto do is keep things fairly simple, so rephrased - is there something like a 'universal' driver board for a class AB amp that people like? If I am asking something that has it's own thread, sorry. I did do a Dyna ST-150 search.
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Old 11th November 2010, 12:42 AM   #7
FrankW is offline FrankW  United States
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Bump - anyone have any ideas?
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Old 11th November 2010, 01:31 AM   #8
sregor is offline sregor  United States
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OK - you might investigate Driver IC circuits, using the LM4702 or LME49810. I don't have any first hand experience, but the circuits are very simple, extremely low distortion. You may be able to find PCB kits which are easily adaptable. Good luck with it.
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Old 11th November 2010, 03:25 PM   #9
FrankW is offline FrankW  United States
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Hi, Steve,

Thank you! A brief look shows these prefer to drive FETs, however, the LME49811 drives bipolars. After being out of diy audio for so long, this is just the direction I was hoping for. Thanks again.

Frank
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Old 16th November 2010, 12:33 AM   #10
FrankW is offline FrankW  United States
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Hi, Steve,

I've decided to go with the LM4702 / Renesas FET path - might as well upgrade the entire signal path!
I'll pop in again when I get it all running.

Frank
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