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AUDIO FORUMS >> D.I.Y. >> 91a Circuit
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Message started by Mannytheseacow on 05/02/23 at 14:23:55

Title: 91a Circuit
Post by Mannytheseacow on 05/02/23 at 14:23:55

I've been toying with building a pair of 91a monoblocks while I wait for my Decware 300b. Anyone have any experience with this circuit?
I read that the standard circuit is a little noisy and benefits from a choke but not sure how to implement that. Also curious for a recommendation for transformers.
I've seen a few kits but ultimately the part layout in these kits isn't very "zen" and I would upgrade pretty much all the parts they come with anyway so starting from scratch seems like a better option.

Title: Re: 91a Circuit
Post by 4krow on 05/02/23 at 16:52:45

Just to give me a better idea, do you have a link to one of the sites for the kit?

Title: Re: 91a Circuit
Post by Mannytheseacow on 05/02/23 at 18:58:34

Here are a couple options that I have seen, 4k. Not much info in either link though.
http://tubeaudiolab.com/diy-kits
http://www.enjoythemusic.com/magazine/equipment/1006/di_hifi_ladyday_91.htm
The first is very customizable and a pretty good option.
The second is only information, if you go to the actual DIY hifi site the kit for the 98’s page is under construction.

Title: Re: 91a Circuit
Post by CAJames on 05/02/23 at 20:08:02

My interest in the 91 amp is entirely theoretical and historical, not practical. So this is all very much FWIW.

My impression is the big sources of noise are that the historical circuit, with the 310A driver tube, has like 40 dB of gain in the driver stage. This was for way back in the day, long before 2V source components. That really amplifies downstream noise and most people deal with it by changing the driver stage, often going with 6S[N|L]7 or C3g tubes and much less voltage gain. The other thing is the AC heater for the 300B tube, which is a whole topic by itself. Steve talks about it in the Sarah design thread. The obvious alternative would be a DC heater.

As for a choke, do you mean a filter choke in the power supply? If so that is relatively straight forward, there is an IR voltage drop across the choke so you need to compensate with a higher voltage power transformer if you want to maintain the same B+ as you would have with just a cap. That would also allow you to decrease the first filter cap to make it more rectifier friendly. But taken all together this adds up to a major redo of the original 91 circuit if you want to start from scratch.

This is all my impression from reading various online fora and a few books/magazines over the years, so again, very much FWIW.

Title: Re: 91a Circuit
Post by Mannytheseacow on 05/03/23 at 04:43:23

CAJames, your response put my interest in the 91 into perspective. Thank you. My interest is primarily historical. So it seems I should go old school with it. And then you reminded me that Steve has solved the problems with the circuit and I’ll be getting one of those in a few years. Inadvertently, I think you talked me out of it! Lol.
I’m pretty amazed, looking at these kits, and what some 91a replicas cost, that Decware offers the Sarah tricked out at the price point that they do. I’m mean, Sarah and the 91a monoblocks are two different things, but it seems that Decware fixed the 91a problems and offers the improved version for less.
Maybe I’ll just stick to my zkit and listen on. [smiley=peanuts21.gif]