GM70

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Paul Barker
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#46 Re: GM70

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simon wrote: Wed Mar 02, 2022 1:32 pm
Paul Barker wrote: Wed Mar 02, 2022 1:16 pm The elephant in the room for a Sukuma version is I can’t afford the required Iron. I think 10y / 801a is the one valve for two stages in front of gm70 but one word, transformer! Yes romantic to use such valves but don’t be a fool and ignore physics.
Yes the compromises can be significant when costs are considered. I had Bud Purvine wind me some ITs for 10Y to drive 300B and the amp was very good. But 12GN7 as triode with simple LC coupling to copper GM70 with separate PS for driver and output is so much better. And simpler, albeit heading in to boat anchor territory.
OK I’ll bight: Darling Fest Two stages, single power supply, Jeremy Epstein relating. Im paraphasing from memory, but you can verify it reading Jeremy’s write up in Sound Practices. I cant find my Sound Practices CD at the moment. first stage rc decoupled, but when he tried same power supply filtering for both stages. i.e. no further upstream filtering straight single b+ node to both stages. It was better.

Different situation but Im still not fully bought into the power supply seperation. Even thouh I dont doubt youre experience Simon, Im not arguing with you, that happed Im sure and it sounded better Im sure.

But the best ever sound Ive had was a single resistorless capacitorless power supply LR filtered, the R in LR being two shunt valves. A no longer member of the forum, or any other for that matter, who is silent but not silent key heard it and it was concurred. Field coil fullrange drivers on Briggs dimensions baffles (there was a easter hobby group back in the days of Soumdpractices with loads of pictures of Briggs dimensions baffle. It was a thing in their group that those dimensions were special. It worked for us that day. Ive never heard anything like it anywhere in the world, not even Bruce Edgar. But it filled the floor of a pretty big room on my top floor, well if you include the speakers. It was the same one power supply for the whole amp, and that powersuply was the best sounding I ever had. But you couldnt pick it up.

I just dont buy the multiple power supplies, unless they are all faultless. My experience varies.

I can tell Im going to have to recreate it for demonstration purposes. Listen paints a thousand words.

We even tried a Black Gate added where there were no caps, we tried paper in oil where there were no caps, we tried motor run caps where there were none. The magic of the sound was lost, you could hear every single cap, where before, words cant describe it to you, the only thing that would show you it is not a simple intelectual science. Its about trying things and finding where youre mileage settles by youre ears. Itll settle in the most impractical thing you ever built, but the best sounding by miles and miles.
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#47 Re: GM70

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All that said, which is a imprint on my past rolled up behind me to drawer upon. It is looking like the gm70 project will have two solid state bench supplies just for the output stages, two solid state power supplies for each channels VA/driver stages. solid state direct heating all along the line. God knows what oscillations thatll occur and combine inelegantly to modulate the most special frequencies. So it has a great chance of not sounding magic whatsoever, so it doesnt even test any hypothesis. But who knows until they try it. It wont prove anything because frome here I can see its full of bad things. But, it may sound magic, not holding my breath. Im only doing it for convenience. But let’s see!
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#48 Re: GM70

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The biamp by stage power people could say, ah but if you built a load of supplies just like that one it would sound better. Really, ok Ill buy that for now. Prove it to me! Build youre multiple capacitorless power supplies, bring them along and we’ll have a listen together. You could be right! Get started then?
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#49 Re: GM70

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Paul Barker wrote: Thu Mar 03, 2022 9:34 am The biamp by stage power people could say, ah but if you built a load of supplies just like that one it would sound better. Really, ok Ill buy that for now. Prove it to me! Build youre multiple capacitorless power supplies, bring them along and we’ll have a listen together. You could be right! Get started then?
The thing is Paul it's about proving it to yourself when it comes to separation of power supplies

The differences can be subtle with some music and massive with other material. But you start to recognise the difference and the clarity

Some of my amps still have one power supply, but the smoothing is per valve.

In the main I've used valve rectified power supplies but the odd times I've used solid state I can't say it's made a difference that I noticed at the time.
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#50 Re: GM70

Post by Cressy Snr »

I’m in awe of all this no-compromise, steam powered, living room sprawl electronics. It’s great that there a few people, deeply read in strange, out of the way subjects, still at it. More power to them.
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#51 Re: GM70

Post by simon »

Completely agree Steve, this a hobby not a commercial undertaking and Paul's continuing adventures are great to read. Doesn't really matter what the outcome is really, though I suspect Paul wouldn't agree!

Anyway, no problem at all Paul, I make no claims for my GM70 amp, other than it being the best amp I've built by some distance. I heard a clear improvement in detail when I added a separate PS for the driver. Whether that was the separation of PSes, unburdening of the PS for the outputs, a combination of both, or something else completely I don't know. If I had more time I might have investigated, but I don't, and a few years on I still really like it so haven't felt the need to play. Your technically complex PSes are way beyond my understanding and no doubt would sound better. I've probably reached the point where "95%" is good enough for me.
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#52 Re: GM70

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Youre gm70 amps sounds great. Its just youre power supply is not the only way to achieve greatness. Sometimes the interactions between channels and between one stage to the next have positive influence and dont wish to be seperated from each other, but remain together. But when youre playing that game you have to be going from 90% effort on the power supply to 500% effort.

But if it makes youre amp experience sound on a par with what Bruce Edgar used to do with his speakers, but you did it with just one power supply.

Its like mind boggleing how each part of the whole can do that. My obsession is power supply I cant deny. That and hating when I have to use capacitors in the signal section. in 801a Ive just got the pio bypass caps, thats all. I tolerate them, lets say. Only just.

Meanwhile Ive found out loads of people have built gm100 amps ffs. Well, not saying I will but if I did Id go parafeed which Josh did. So theres a whopping great big cap!

In short theres no rules, do what makes sense for each individual scenario!
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#53 Re: GM70

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Agree that no capacitor psus sound best.. following Steve Bench,s VR based psu I did the same and found it better that a capacitor based psu by a significant margin but not better that an active shunt regulated psu... hence my championg of shunt regulated.

The big thing seems to be ato keep the audio signal out of the psu. Conventional design does this vis a bypass capcitor and most psus do not have a sufficently large or good enough capacitor to do this for all audio frequences and this is were ultrapath helps but either way the signal passes through a capacitorand hence adoprs the capacitor sonic signature to some degree and sually that is more than a good shunt device...

Wether one can be bothered with the incresed complexty is another matter....

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#54 Re: GM70

Post by Nick »

I found that keeping the higher frequencies out of the regulator was most important. Hence my small resistor 10R or so and a good quality 4u7 or larger film cap on the end of the supply.
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#55 Re: GM70

Post by JamesD »

Agree with that finding... Most of my shunt designd have a small cap on the output... Adding the resistor is a improvement so thank you I will adopt that :-)
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#56 Re: GM70

Post by Nick »

Yep, it hits the Rout of the supply but we are talking about valve impedance here so 10R is no big deal.
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#57 Re: GM70

Post by Nick »

I think Mark uses a nice shunt supply if you want to go the valve route. Solid state shunts are great, but maybe not for power stages as the dissipation starts to be problematic. TBH for me now I have moved away from valve output power stages, even ignoring the cost and lack of availability any more (I was told the other day Brian Sowter had retired, so thats one less, still being made but elsewhere I believe), the damage the output transformer does cant be ignored. Its hard to fight something as simple as the MoFo if you want added 2nd harmonic.
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#58 Re: GM70

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simon wrote: Thu Mar 03, 2022 1:26 pm Whether that was the separation of PSes, unburdening of the PS for the outputs,
I think so, its amaising how a slight lightening of the current demand on a power supply suddenly makes it sound relaxed and shapes the whole sound much more delicately, finely, and kicks ass in bass aswell, can be a miniscule change in current too, Ive heard light and day differences from just lightening the load of the single power supply by 15mA. A stressed power transformer sounds terrible and it doesnt take much to go into saturation.

Its not the only thing going on, but going forward, for me is my only angle. In youre world multiple supplies do sound absolutely great for you and I loved the sound of youre GM70 in youre home with all the other parts of the chain yours, which is the best way to evaluate.

With an LR power supply the shunt valves pull at least the same quiescent current as the signal section. The power transformers rectifiers and chokes must not be stressed by that huge excessive current demand. So it has to be far in excess in each of its components than a bunch of small supplies with combined half of that duty, unless you too have shunt regulation.

All I can say about the two paths is that a single supply of this quality is the best thing Ive ever heard.

I havent invented anything I just trusted Stephie. Or should I say tried what Stephie did and found it right for me.
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#59 Re: GM70

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Nick wrote: Thu Mar 03, 2022 4:41 pm TBH for me now I have moved away from valve output power stages, even ignoring the cost and lack of availability any more (I was told the other day Brian Sowter had retired, so thats one less, still being made but elsewhere I believe), the damage the output transformer does cant be ignored.
That would explain why my last pair of Sowters came from Carnhill…..
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#60 Re: GM70

Post by Paul Barker »

Great, found my second of the bought new pair gm70 plus a bonus pair of new copper gm70’s I don’t remember buying! So now I have 6 copper gm70s, two of those used tested good cryi-treated (if that sounds different we’ll find out.); If it sounds better enough.. a big IF! Then I’ll be googling valve cryo -treatment, maybe it can be done with dry ice, no idea as of yet. If they are remarkably better …. A bigger IF? I’m sure there’s a facility you can send them for cryo!

Bases still at Russian Port, who knows if they’ll leave port for UK?

+ 18 new graphite gm70’s

I think that stash will outlive me. An amp I won’t mind using daily.
Last edited by Paul Barker on Thu Mar 10, 2022 8:10 pm, edited 3 times in total.
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