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GM70 Triode Stereo Oddwatt SRPP / SIPP
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Author:  cheap-Jack [ 01 Jun 2010, 14:42 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

Hi.

Questions per yr schematic:=

(1) Show me the grounding schematic of signal reference ground & the chassis ground considering
digital heating of the power tubes.

(2) What HV wires you would choose for 1KV plus HV feeding?

c-J

Author:  sampleaccurate [ 01 Jun 2010, 19:37 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

cheap-Jack wrote:
Questions per yr schematic:=

(1) Show me the grounding schematic of signal reference ground & the chassis ground considering
digital heating of the power tubes.

(2) What HV wires you would choose for 1KV plus HV feeding?

My digital camera batteries died, but when they recharge I have some photos I want to post of the RFI on my amp outputs.

One, the amp itself has very little RF noise on the speaker outputs with no speaker hooked up. BUT, hook up a speaker and the noise jumps through the roof. The speaker wire is acting as an antenna and pumping noise INTO the amp - the amp is not sending RF to the speaker, or if it is, it's at a far lower amplitude than the noise picked up by the speaker leads. The measurements were taken at the 4 ohm secondary with the speaker connected and disconnected and the input of the amp grounded.

Second, with the speakers disconnected so that no external RF is being picked up by the speaker wires, the amplitude and spectrum of the RF noise looks IDENTICAL when the power is switched off as when the power is switched on. The SMPS units are off, the diodes aren't switching, the transformers are not energized, but the capacitors keep the amp running. Not only does the amplitude of the RF stay the same, it persists after the caps have run down and the amp is completely discharged. In other words, about 100% of the RFI I'm measuring is coming from sources external to the amp - not SMPS units, and not "sand" diodes.

Based on this one test IMO your concerns about SMPS generated RFI are overblown. A choke on the speaker leads (antennae) would probably do more to get rid of RFI than eliminating a SMPS. I think you're pointing the finger at an innocent party.

Author:  cheap-Jack [ 02 Jun 2010, 08:59 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

Hi.

(1) For digital powering audio amps, elaborated RFI grounding device is needed to drain RFI out the amp. FYI, I've installed them in ALL my sand & tube phonostages & power amps though no digital powering is used in any of them.
I have done so to drain off any airborne RFI emitted from all those digital gadgets at home, e.g. cellphones, cordless phones, WiFi, PCs,.LCD/plasma TVs, camera chargers, major appliances with built-in digital controls, etc etc etc.

We are now living in a digital era. But our analogue audio amps are designed decades before our time. We got to do something about it.

(2) Standard hook up wires are rated 300V to 600V. For use over 1KV with those wires is asking for trouble, e.g. electrocution or fire hazards.


I highlight the points to be discussed is to let our readers to focus on them without need to read the whole post again & again, wasting the forum bandwidth & time.

If you don't appreciate my goodwill to help, don't you bother answering. But don't forget yr posts are property of this public forum & are intended to read by tons of readers wherever. I believe it is the posters' job to let our readers to get the right thing.

c-J

Author:  cheap-Jack [ 02 Jun 2010, 09:52 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

sampleaccurate wrote:
Based on this one test

Hi.

What equipment you used for yr test?

sampleaccurate wrote:
SMPS generated RFI are overblown

Prevention is always better than cure. May I suggest you to read more about RFI & you will appreciate why we should be more proactive than reactive on this everyday digital issue.

sampleaccurate wrote:
hook up a speaker and the noise jumps through the roof. The speaker wire is acting as an antenna

So you already confirmed RFI is everywhere & picked up by loudspeaker cables.

I have seen major audio/video manufacturers have the loudspeaker cables supplied free with their auido products all get RFI ferrite suppressor ring molded on. To kill RFI for sure.

I know a brandname loudspeaker cable maker supply hi-end loudspeaker cords with a special inline conditioner which is claimed to make sound much better. RFI killer I suppose?????

So if a cheapie RFI ferrite ring can kill RFI picked up by loudspeaker cables. Why worry too much about it? Loudspeaker cables are PASSIVE but audio amp is ACTIVE which handles small audio signals. Any RFI contamination can modulate the music signal & affect the sound.

I think out a cheapie simple solution to heat up GM-70s without using any SMPS or even costly custom-built filament power transformers. Hint: No AC heating, So no hum. Guess what it is?

c-J

Author:  sampleaccurate [ 02 Jun 2010, 12:20 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

cheap-Jack wrote:
So if a cheapie RFI ferrite ring can kill RFI picked up by loudspeaker cables. Why worry too much about it? Loudspeaker cables are PASSIVE but audio amp is ACTIVE which handles small audio signals. Any RFI contamination can modulate the music signal & affect the sound.

Not worried about it. What you're missing is that the speaker outputs feed directly back INTO THE INPUT OF THE AMP THROUGH THE FEEDBACK RESISTOR AND CAPACITOR.

So the RFI that comes in your speaker leads gets sent DIRECTLY to the sensitive, high gain input stage to get amplified/intermodulated with your signal source.

I think a ferrite RFI suppressor on speaker wires is a great idea. I think going to the trouble of heating tubes with batteries is a waste of resources that could be devoted to more effective ways of suppressing noise.

I think I know a lot more about RFI than you think I do, and I think you know a lot less than you think you do.

I used a scope BTW. Ask me the bandwidth.

Author:  cheap-Jack [ 02 Jun 2010, 13:19 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

sampleaccurate wrote:
I think I know a lot more about RFI than you think I do, and I think you know a lot less than you think you do.

Hi.

Nobody knows everything. I don't & I read a lot to get to know more.

(1) Tell me what is a RFI grounding device which I know its importance & already installed in all my audio amps? Please don't tell me I know more than you do.

(2) If you so worry about loop FB which provides RFI FB from loudspeaker cables, why not design yr amp of the right gain without need of using loop NFB?

Loop FB should always be avoided to prevent an amp going into transient instability, RFI existinng or not.

c-J

PS: Yes, you are fhe first person to alert me about the loudspeaker cabling can be such a severe source of RFI cause by loop NFB.

Author:  sampleaccurate [ 02 Jun 2010, 15:19 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

cheap-Jack wrote:
why not design yr amp of the right gain without need of using loop NFB?

I just built a 300B SE amp with ZERO feedback. There's no direct path from the speaker leads to the amp input.

To make matters worse, consider also that the feedback capacitor becomes a short ciruit at radio frequencies (excepting resonant points from self inductance), so you might as well bypass the feeback resistor with a wire as far as RFI getting into your amp input stage from your speaker "antenna" is concerned. The RFI on the speaker leads sees very little impedance back to the amp's input stage.

I'm investing in some snap on chokes first to see what they do if anything. If that doesn't work I have some other ideas, starting cheap and working my way up.

I have to admit I'm amazed at how much garbage is picked up by the speaker leads. I'll try to measure the RF noise at the feedback node itself tonight on the other side of the feedback resistor and cap to see how much actually makes it to the amp input.

I don't know it all either c-j, I read and experiment and learn like you.

Author:  sampleaccurate [ 04 Jun 2010, 20:35 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

I found some small ferrite cores (I think they're ferrite) that I used in constructing an RF linear CB amplifier once a couple of decades ago, so I wrapped the ends of the speaker leads around them about 10 times and the RF junk on the speaker terminals is noticably attenuated. I suspect if I got the right chokes that it would be even more effective.

I also updated the schematic for this project below. In addition to that I found out Edcore will custom design a transformer for $40 (which is insanely low if you ask me) so I'm going to have them design and construct the best thing they possibly can.

Image

Author:  sampleaccurate [ 04 Jun 2010, 23:35 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

Time for a little humility!

I've been doing more calculations. Reading, calculating, reading, calculating...

I now see what the problem with my amp design is. It's the B+ voltage required because of the tube I chose. I'm looking at potentially putting up to 4000 volts peak to peak across the primary of a 20K custom tranny to get 100 watts! WOW! The even bigger problem is the increased B+ required due to the plate characteristic curves of the GM70. The tubes have two problems: high plate impedance requiring a high impedance transformer requiring a high AC voltage for rated power; and, the zero grid voltage plate curve intersects any reasonable load line at several hundred volts (up to 500 volts) which increases the B+ that's needed.

The 10K Edcor model CXPP100-MS-10K would need only 2800 volts peak to peak for 100 watts assuming a sine wave input. My design would allow 2300 volts swing and used with that tranny would yield about 66 watts, but the tubes would see a low impedance load they wouldn't like and linearity would suffer.

However, I can't even do that.

The the zero grid bias plate curve is at 450 volts where the load line intersects, which forces B+ way up beyond what other tubes would require. There's a 450 volt "window" where the tube is non-conducting, pushing up the required B+ to unacceptable voltages. I need a transformer that can handle a very high voltage. I'm learning a lot!

When I talk to Edcor I now know a lot more about what I need and what the problems are. Now I see the reason for no secondary taps and no UL taps. Bringing out those taps probably reduces the breakdown voltage.

I'm "getting" this now I think. It will be interesing to see what Edcor can do. I'm thinking the lower the impedance the primary is the less voltage I'll need to put across it and I can drop the B+ a little. Maybe a 15K tranny with no secondary or UL taps could be made to withstand a lower B+ and still extract a resonable amount of power from the tubes. I'm running some calcs now to see what B+ would be needed for 50 watts at 15K using a GM70.

If Edcor can't get me the B+ required for 50 watts at 15K I may change my tube selection. $170 mistake counting the sockets. I could unload them for $100 on ebay and lose $70. Fortunately the power tranny has a half voltage tap and can be bucked by a small transformer to make another amp with different tubes. Maybe like a parallel pair stereo KT120 oddwatt?

Save me Edcor!!! :xfingers:

If they can't it still appears I can go single ended and get 28 watts with a Hammond 1638SEA. It's specifically made for triodes, has no UL taps, and is hi potted to 2000 VRMS (over 5600 volts peak to peak).

Does anybody see a problem with that (other than the public humiliation of changing my design!)? :blush:

Author:  sampleaccurate [ 05 Jun 2010, 00:59 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

From the latest calcs I've done with a B+ of 1150 volts, what I originally intended to use, I could get about 65 watts of output from a PP pair using a 15K primary transformer. If Edcor can build a tranny that can take a B+ of 1150 v with a power capacity of 65 watts and an allowable bias current of 100 mA AND my calculations aren't wrong I'm in business.

I also came to the conclusion that the GM70s aren't capable of any more than about 75 - 80 watts PP with a 15K tranny - period. If I'm correct the voltage simply can't be developed. If I take the B+ any higher I risk damaging not only the tranny but the tubes, although there's debate about how high I can go with the tubes.

There's also a Hammond model that will allow parallel SE operation of two GM70s for a whopping 50 watts of SE triode power. Unfortunatley it weighs 30 lbs. (60 lbs. a pair) and costs over $700 a pair so forget it. :(

I'll probably discover this is all wrong tomorrow. :smoking:

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