GM70 PP

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izzy wizzy
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#196 Re: GM70 PP

Post by izzy wizzy »

It's been a while and apart from it being a bit noisy, I'm very pleased. However it's never been totally quiet and I'm not 100% sure about the SMPS filaments for the GM70s; that being a bit of a compromise.

So first bought Hammond transformers for this based on Rod Coleman's design for his regs https://docs.google.com/document/d/e/2P ... vGjfcE/pub but upped the tx recommendation by 2V to enure decent drops across components in modded Lehanne DHT VCCS regs. It still buzzed but in a different way.

With a bit of isolation of supplies, found the source in the filament supplies. After a lot of head scratching, decided to earth all the iron frames that were mainly Hammond transformers. They don't have any earth arrangement as I guess they'd normally be bolted to a metal chassis. Mine is all wood.

With just the output stage powered and the interstage earthed on it's input, finally, for the first time ever, it was silent. Woohoo!

Unfortunately, there's some noise in the first two stages so I'll have to repeat the isolation efforts here as well and hopefully get the same result.

It did make me wonder about when others have had noise issues on breadboards as to whether they might be having similar issues due to the non earthed iron.
Max N
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#197 Re: GM70 PP

Post by Max N »

I have found the same thing - although in my case the issue wasn't necessarily audible, sometimes it was just showing up on a scope, in which case it was very easy to see the benefit.
Might also be a safety thing?
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#198 Re: GM70 PP

Post by simon »

izzy wizzy wrote: Fri May 12, 2023 5:45 am So first bought Hammond transformers for this based on Rod Coleman's design for his regs https://docs.google.com/document/d/e/2P ... vGjfcE/pub but upped the tx recommendation by 2V to enure decent drops across components in modded Lehanne DHT VCCS regs. It still buzzed but in a different way.
Good choice - I found the recommended trafo only gave me a max of 19.5V, not really a problem but it always feels more comfortable to have a little headroom.
izzy wizzy wrote: Fri May 12, 2023 5:45 am It did make me wonder about when others have had noise issues on breadboards as to whether they might be having similar issues due to the non earthed iron.
Hmmmmmmm.......
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izzy wizzy
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#199 Re: GM70 PP

Post by izzy wizzy »

I've finally done what I set out to do 20 odd years ago. The amplifiers are now quiet enough and so I can finish the case and think about improvements now the technical basics are in place. There were times when I thought of giving up. Going to the shed to try and get previous versions going was a traumatic and miserable experience. But going public with the development here on this forum has allowed the design to work out. The end result is better than I could have imagined.

The Lehanne DHT regulators work brilliantly. The one I modified to work for GM70 seems to also work well. Hindsight is a wonderful thing and I should have gone for linear supplies to heat the GM70 rather than SMPS. I couldn't get the SMPS noise out and going linear has dropped the noise floor significantly. Using the Coleman design to feed the Lehanne regs is probably the most significant improvement I've had.

So thankyou to everyone that contributed to this and the previous thread (the 813 PP amp). I wish I could bring the results to Owston but it's a bit far. The exposure to the kit, sounds and people I experienced the time I did go was instrumental in keeping going and a driver to getting the success I have.
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#200 Re: GM70 PP

Post by simon »

Congratulations Stephen, it sure sounds like a landmark. But what are you going to do with your time now? :lol:
izzy wizzy wrote: Sun Jun 25, 2023 2:41 am Using the Coleman design to feed the Lehanne regs is probably the most significant improvement I've had.
Do you mean you're using Rod's DHT reg boards to power Andrew's? i.e. nested regs? I've not heard of anyone doing this.
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#201 Re: GM70 PP

Post by izzy wizzy »

Only using Rod's raw dc circuit to power a modded Lehanne reg.
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#202 Re: GM70 PP

Post by simon »

Ah right. Me too.
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#203 Re: GM70 PP

Post by IslandPink »

Good effort and glad it worked out so well for you.
"Once you find out ... the Circumstances ; then you can go out"
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izzy wizzy
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#204 Re: GM70 PP

Post by izzy wizzy »

simon wrote: Sun Jun 25, 2023 10:30 am Congratulations Stephen, it sure sounds like a landmark. But what are you going to do with your time now? :lol:
I now have to make them look good or at least acceptable for living room :roll:
SWMBO has been very understanding so far but .....
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#205 Re: GM70 PP

Post by izzy wizzy »

Couldn't resist trying something that I'd found to be good in some of the previous failures; direct coupling stage 1 and 2. Not my idea but a DHT version of Kevin Carter's front end to his amps.

Needing more HT, I made the front end supply cap input with the max cap value for the DW4 rectifier of 16uF.

GM704P1L2P29LAud2.gif
GM704P1L2P29LAud2.gif (25.03 KiB) Viewed 4126 times

SWMBO wondered if she could hear the difference in all the repetition going on in the back and forth between modded amp and the standard version. Played in mono. Her description was you could follow the music better with instruments sounding as they occupied their own space rather than a bit homogenous in comparison. But is was a bit sharp. Couldn't have put it better myself.

So made the second channel the same and listened for a week. It was a bit sharp but very engaging. Over articulated maybe? Who knows. A bit wearing but ignoring that, addictive.

Was a bit concerned I had a cap input supply and so revised it back to choke input, 5R4GY rectifier to allow higher PIV, hybrid bridge with IN4007 for the negative bit and used another multiwinding transformer to buck the HT on the primary, thanks Nick, to bring the HT down from 500ish volts to 370V. The super etched sound is gone that was bothering. Was it the cap input supply or all the other changes? Again, who knows but I'd wager it was the cap input in an unregulated supply from previous similar efforts.

As a first go for this topology it reminds me of the spacious and open presentation when it was a 2 stage amp with the vividness of the 3 stage. It's also quieter.

The Lundahl 1635 is an old IT design and the 1660 was going to go in but wanted to try this first figuring no IT should be better than a good one.
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