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Old 4th March 2011, 06:07 PM   #1
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Default KTA-HiFi ST-70 pcb or marantz 8b. HiFi build

I realize that being a newbie my first post was quite generic in my question of amp suggestions. So iv done a little reasearch and have decided id like to attempt to build an amp from the marantz 8b schematic or one from the kta- hifi pcb. I like the idea of building from a pcb but iv heard the st-70 is a little lacking in sound quality. Although parts wouldn't be such an issue. As far as the 8b gose I guess my question is.. am I going to be hard to find the parts I need? I hear that the 8b has excellent sound.
Any help is apreciated. Thanks, paul
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Old 4th March 2011, 07:46 PM   #2
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Marantz 8b used somewhat non-standard transformer which has tertiary winding for NFB. You can try mod developed by Japanese engineer (sorry, but I forgot hes name) with standard PP transformer.
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Old 5th March 2011, 12:49 AM   #3
kevinkr is offline kevinkr  United States
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Are you planning to build an ST-70 type amp using chassis and transformers from someone like dynakit? If not there are certainly other transformers that are significantly better performers than the stock Dynaco transformers.
In triode connection though they are surprisingly decent performers, and the amplifier sounds quite nice.

The parts required for the driver board are readily available from Mouser, Digikey, and Michael Percy audio consultant. I recommend metal film resistors and good coupling caps. (Clarity ESA, REL PPFX or better, etc.)

Power supply improvements are always worthwhile as well.

Yeah, I'm the guy behind KTA. (The board is licensed to Geek who currently offers it.)
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Old 5th March 2011, 04:20 AM   #4
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Kevin,
Wow no one better to talk to than the guy who designed the board. Ill actually be fabricating my own chassis as I'm a metal fabricator by trade and it will be no problem to get a hold of a good sheet of aluminum (I'll probably frame it with wood). I would like to upgrade as much as possible (as cost allows). The only thing I'm concerned about is the output in triode operation. Thats about half the power of the regular operation right? the speakers I'm building this amp for ask for 50wpc. They are actually very efficient for their size (Frazier sevens).. I might need more power..

Last edited by iglehart332; 5th March 2011 at 04:28 AM.
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Old 5th March 2011, 12:28 PM   #5
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The specs of the speakers state they will handle 50 watts pink noise continuous with I believe 125 watt peaks. They are 99db 1 watt 1 meter efficient.

You do not need 50 watts to run them. a Dynaco stereo will do fine.
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Old 5th March 2011, 12:40 PM   #6
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quote:
Are you planning to build an ST-70 type amp using chassis and transformers from someone like dynakit? If not there are certainly other transformers that are significantly better performers than the stock Dynaco transformers.
In triode connection though they are surprisingly decent performers, and the amplifier sounds quite nice.


I have the opinion that the transformers on the Dynaco stereo 70 are fine. One thing about this hobby is that you can get hung up on spending way more money than you need to to achieve a desired goal. The dynaco transformers perform well. Is it worth spending hundreds of dollars extra on the gamble that it might perform better sound better? In my opinion one has to draw a line somewhere. Triode in my opinion is a myth. It sounds ok as is and maybe just maybe a touch sweeter in triode depending on the program one is listening to. Install a quad pole double throw switch in the mix so that you can run it either way. Do not however flip the switch when the amp is powered up.

As one who has used a number of different driver boards I would highly recommend the Dynamutt or the Mapletree driver board over the KTA board. Yes, I have used the KTA board also.
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Old 5th March 2011, 07:58 PM   #7
roline is offline roline  United States
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If you want triode power don't forget Pete Millet's amp. 20 to 250 watts channel depending on iron and tubes chosen. I've just fired my my second compactron amp, kinda simplified version of Pete's. Its set up for 40 watts/channel triode mode, and it can chase you out of the room........ Pete's board is around $50 and gives Dynaco a run for the money!! The ST70 can be heavily modified, power supply, driver board, triode mode, don't forget different iron choices....All it co$t$ i$ a little $$
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Old 5th March 2011, 09:05 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by roline View Post
If you want triode power don't forget Pete Millet's amp. 20 to 250 watts channel depending on iron and tubes chosen. <snip>
This ought to be a pretty good option as well. Pete's stuff tends to be well thought out, and the boards are well made.. (I've not tried this particular board)

The KTA design is over 23yrs old now and hasn't changed at all in that time, it's a bit old school in terms of its design and was designed at a time when the supply of better suited small signal tubes seemed questionable. (Pre-internet) Assembled boards came with Solen coupling caps as a result of needing to keep costs low - these more than anything else could stand replacement with something better. (The assembled and tested board sold for just $79 in 2001 the last year I offered it directly.)

IMHO I found the difference between A-431 tOPT riode connected as opposed to UL to be quite significant and I strongly preferred triode connection. This may be a function of what I was listening for at the time as well as the speakers I was using. What little I remember of actual measurements was that the distortion spectrum in triode mode was much less extended than in UL mode.
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Old 6th March 2011, 12:51 AM   #9
dhieber is offline dhieber  United States
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I have your board Kevin and IMHO it is the best sounding of my three modified ST70s. I have, however, swapped the LTP tube for a 12BH7 which works well in this position. What caps would you recommend as replacements for the original Solen coupling caps?
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Old 6th March 2011, 01:07 AM   #10
taj is offline taj
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Quote:
Originally Posted by iglehart332 View Post
but iv heard the st-70 is a little lacking in sound quality.
I just want to make sure you realize that that commonly held opinion is referring only to the stock original circuitry, which is completely replaced by all the various driver boards available -- including Kevin's KTA board. The reason they all exist is due to that opinion, and the fact that the rest of the components form a pretty decent foundation to build upon.

They all sound different from the original ST-70 circuit, and from each other. The only thing common to them all (or at least most of them) are the power supply and output sections.

Have fun.

..Todd

Last edited by taj; 6th March 2011 at 01:12 AM.
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