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GM70 SE amp project
GM70 SE amp project
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Old 17th November 2020, 10:38 AM   #21
Rod Coleman is offline Rod Coleman  United Kingdom
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GM70 SE amp project
If you mean filters made with common-mode chokes and "Y" Capacitors (line→safety earth), I think they are not necessary, and are liable to degrade the noise performance.

This kind of filter is produced to address some of the problems of mains→DC coverters, in order to meet the statutory EMC regulations (CISPR22 - EN55022 for IT gear for example). They are intended to reduce the portion of noise from mains→DC converters from getting back into the mains.

But if you have a transformer, with low PRIMARY→SECONDARY capacitance, the insertion loss (noise blocking) is already excellent, especially in the range 1Hz to 10MHz, where filtering requires very large components.

CM choke filtering often uses mH-region inductance values, and this reacts with capacitors to produces big resonance peaks in the impedance plot, often at quite low frequencies.

Meanwhile, the differential inductance of common-mode chokes is excited by the pulse currents of tube amp DC supplies, and generates noise of its own.

For mains isolation and noise control, a good quality mains trafo is needed. If you have one with an interwinding screen, there is no need for input filtering, other than an "X" cap across the Line-Neutral at the input.
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Old 17th November 2020, 11:58 AM   #22
gideon1990 is online now gideon1990  Netherlands
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Okay nice to see my line of thought about this confirmed
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Old 17th November 2020, 06:20 PM   #23
audiowize is offline audiowize  United States
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Quote:
Originally Posted by Rod Coleman View Post
The problem arises when there is a continuous ground connexion between the DHT amplifier and a DAC, or if the DAC is close enough to the amplifier to pick up EM interference.
This of course assumes that the DAC doesn't also contain a SMPS.
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Old 17th November 2020, 06:51 PM   #24
bobikdartanan is offline bobikdartanan
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I'll add a little.
Not audible high-frequency noise and high-frequency harmonics of the amplifier also affects the subconscious, which leads to quick fatigue of the listener.
And the quality of the audio system is determined by the listening time without signs of fatigue.
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Old 17th November 2020, 07:50 PM   #25
bobikdartanan is offline bobikdartanan
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Sorry, I forgot the main thing:
SE tube amp without NFB, in comparison with transistors amp, usually, but not always, has a rapidly decaying spectrum of high-frequency harmonics, as well as THD corresponding THD of the ear:
https://citeseerx.ist.psu.edu/viewdo...=rep1&type=pdf
Switching power supply deprives SE tube amp of the main advantage.
as well as diodes in the rectifier:
Diodes

Last edited by bobikdartanan; 17th November 2020 at 07:53 PM.
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Old 24th November 2020, 01:20 PM   #26
manojrc is offline manojrc  India
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Can someone please help me to get GM70 tube datasheet?

Thank you..
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Old 24th November 2020, 02:01 PM   #27
Stephen R is offline Stephen R  England
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I used the Meanwell supplies to heat my PP GM70s. Even after full turndown, there was still too much voltage so I burned off some with series resistance and then fed the fil via a CMC, no cap after. It was OK but later (now) I used a VCCS regulator after (Rod's would be another example) and it improved things quite a bit. Would a linear supply be better? I don't know, maybe, but I haven't the funds to find out and the amp sounds plenty good as is.

I would be concerned about the grid resistance for fixed bias in the GM70. 30K is recommended maximum for fixed bias. With 300k you might risk runaway. A 30k grid resistance would be hard for your driver and makes cap coupling very hard. You might need a rethink in this area.
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Old 24th November 2020, 09:37 PM   #28
PRR is offline PRR  United States
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Quote:
Originally Posted by manojrc View Post
Can someone please help me to get GM70 tube datasheet?
A modern 1-page sheet:

https://frank.pocnet.net/sheets/084/g/GM70.pdf
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Old 25th November 2020, 05:30 AM   #29
Flikoman is offline Flikoman  Croatia
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Quote:
Originally Posted by Stephen R View Post
I used the Meanwell supplies to heat my PP GM70s. Even after full turndown, there was still too much voltage so I burned off some with series resistance and then fed the fil via a CMC, no cap after. It was OK but later (now) I used a VCCS regulator after (Rod's would be another example) and it improved things quite a bit. Would a linear supply be better? I don't know, maybe, but I haven't the funds to find out and the amp sounds plenty good as is.

I would be concerned about the grid resistance for fixed bias in the GM70. 30K is recommended maximum for fixed bias. With 300k you might risk runaway. A 30k grid resistance would be hard for your driver and makes cap coupling very hard. You might need a rethink in this area.
Fixed bias via interstage transformer

I use SMPS (24V 4.5A) slightly modified, recapped and RCRC to get 20V. I am very happy with it and it works without issues 5 years now.
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Old 25th November 2020, 08:09 PM   #30
deafbykhorns is offline deafbykhorns  United States
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Quote:
Originally Posted by manojrc View Post
Can someone please help me to get GM70 tube datasheet?

Thank you..
I have one I can take a picture of if you can read Russian
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