Germanium Amplifier

For the three and more legged things
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thomas
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#136

Post by thomas »

Mmmm interesting!

Always been a bit disconcerting to me how volume pots can sound different- haven't found one yet I've been completely happy with. Might have to try one out for myself...

Ta for the heads up!
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colin.hepburn
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#137

Post by colin.hepburn »

Great stuff mike nothing wrong with a good SS designed Amp 8) :)
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Mike H
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#138

Post by Mike H »

Yep. :)

thomas wrote:have to try one out for myself...
For 20 quid worth a sperrymint

Isn't it funny, I was having this brief conversation with Nick at Owston sort of along the lines of "what's the point of golf" (Owston Hall hotel has a gold course) and I said I heard somebody say (subject was just mentioned in a radio 2 proggy recently) that no two games are alike, and each time it's mostly about trying to get your act together.

So say you had a really great game one day you'd be like "can't wait to get back to the course and do that again cos it was brilliant", but, next time you can't drive properly, can't putt properly, ball just won't go where you want it to...

Similarly, couldn't recapture the previous evening's 'greatness' last night, my hearing was just 'not in the mood' :shock:
 
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Mike H
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#139

Post by Mike H »

One more piccy I've just found in the camera ~ moved IT plus few comp changes on board. Plus Zobels on the o/p terminals in the background.


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#140

Post by Orit »

Hello Mike,

I've just finished reading this thread, and I'm feeling happy I'm not the only Martian who likes the sound of this type of amps.
I collect early transistor hifi sets, so it doesn't surprise I've got four stereo sets with transformer-phase splitter Germanium output stages:

A 1964 Harman Kardon SR 900 receiver,
a 1965 ElectroVoice EV-1177 receiver (same as the 1144, but with an FM tuner aside it),
a 1967 Philips 22GH925,
a (don't know the year) Panasonic SG-999 music center.

All of them have that particular "full" sound - which can be 2nd harmonic distortion - and terrific dynamics, probably a result of low over-all feedback ...
I particularly enjoy listening to the ElectroVoice, though it's humming a bit: the HK should be better, but it has some faults, and the original O/P trannies have been replaced in the past with AD149's, which is wrong for +/-23V power supply I think ...
Thank you again

Marco

P.S. Have you tried your amp with Bryan Adams's hits ? 8)
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Mike H
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#141

Post by Mike H »

Wow thanks for that :D

Haven't knowingly played Bryan Adams unless he's been on Radio 2 :lol:

I found a schematic for the HK, the way the driver is configured is same as an earlier version I was working on ~ where IT is not gapped ergo it is driven 'parafeed' from the TR collector, which has a 1k collector resistor, and other end of primary is AC grounded through 100uF. The bias for same transistor is then taken from that point, which of course = the no-signal collector Voltage.

Dunno how mine compares to vintage designs, as has got modern passive comps and IT's, but after getting cat cables and impedance correction networks on the speakers, and with good quality vol pots on the input, sounds pretty clean to me. :thumbleft:
 
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#142

Post by Mike H »

Thinking about what DTB and Nick were saying about this over the Owsters weekend, I had a listening sesh last night after getting home from Owsters yesterday and I thought hmm yes maybe treble is a mite subdued.

Out with soldering iron and odd tools and a few tweaks later R9 is strapped with 100uF. This had the effect of reducing the residual output hum by half. This is encouraging since as it's mostly picked up by the splitter/driver transformers, this implies that the increased open loop gain this has enabled allows the negative feedback to take more of this out.

In the same way it seems to make it more HF stable for the same reasons (more open loop gain = tighter control) so then C7 could be reduced to 10nF.

Better!

Still not sure what is meant about the 'weak bass', or what's causing it, whether I can do anything about it, or if I care :D

Although the o/p cap is quite small value at 1,000uF, most likely that.
 
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Dave the bass
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#143

Post by Dave the bass »

Yeah, it weren't a 'diss', honest! Just a comment :)

To me it sounded limited in bandwidth at both ends. Thats what robbed it of life and drive IMO. Maybe that nice 801a amp of your's has nicked all the soul and life out the Geranium (sic) amp!!!

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#144

Post by Mike H »

You'd think so wouldn't you? :D

Well I don't know. Got to the point where rather than poking and guessing maybe I should get the test gear out and look at it properly. So I did.....

Bypassing R9 was a bad idea, very HF unstable like that and keeps wanting to break into oscillation at 50 kHz -ish. Even if the vol. pot is wound up just a bit more than 'normal' living room level. The transformer is resonant around the 50 - 60 kHz area. Once 'triggered' it will keep going, at one point one half of the heatsink was getting very hot while that amp was pumping out several Volts of ultrasonic. Square wave very 'dirty' with excitable trains of oscillation on the horizontals, although it appears 'clean', that is, one frequency, unlike the hash of noise I was getting last time I was making it quiet.

I also took off the Zobel networks from the outputs as they seemed to be aggravating things actually. This might mean however that the amp is possibly unstable without speakers connected.

So tried reverting to original circuit values (but without the o/p Zobels). Viz R9 unbypassed and C7 = 47 nF.

On the 'scope measures as -3 dB @ 20 Hz, and -3 dB @ 35 kHz. Which I think is not too bad really, so dunno what youse all is on about. :D

OK how small can we go on C7 ~ for 10 nF (last night's experiment), +3 dB @ 40 kHz, bit too high. However still keeps transformer resonance in control as it goes down after that.

Best optimum so far, C7 = 20 nF, 0 dB @ 30 kHz and -3 dB @ 43 kHz (roughly).

Note for Andrew L, when it was on your test rig last Owston you noted the ultrasonic slope off and corrsponding increase in distortion, well it would do that :D once past the resonant point the transformers run to zero at about 100 kHz, so that doesn't surprise me. :D

And indeed, that's what the amp does now.
 
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#145 Germanium Amp Revisited

Post by Mike H »

Germanium Amp Revisited

This below ... wow it's now 12 years old! 12 years ago I made this...
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Germanium Amp 2024-01.jpg
Germanium Amp 2024-01.jpg (172.96 KiB) Viewed 515 times
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It's been to Owston a couple of times and got at least one or two favourable comments as I remember. However it always had a few issues. I was looking at it again recently (for no obvious reason) and ended up making the following notes on the schematic -
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Germanium amp 2012-4D.png
Germanium amp 2012-4D.png (144.21 KiB) Viewed 515 times
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1. "this is cobblers"
The front end, all by itself, must have its own dedicated regulated supply else it's unstable and oscillates, and just fiddling with the supply is not really the way to fix it. Ideally, it shouldn't care. LTspice confirms the behaviour if I remove the fixed Voltage source (V2) and try it through a dropper resistor. Seems that the issue is with R6 and R13, especially R6, which connects Q2 emitter to the transformer primary (seemed a good idea at the time, as a form of local NFB), so get rid. Also, reduce the open-loop gain (use lower value resistors). Also, lose the MOSFET which I think is just confusing the issue. Just make it simpler. It is then stable (in simulation anyway) and so can be powered through a familiar RC filter from the +Vee rail off the o/p stage (100 Ω and C5), without it having a tantrum.

2. "Do this better"
The phase-spitter transformer and the o/p stage combined also contribute to an instability issue, where they don't like a high impedance or open-circuit on the output, if so, HF oscillation occurs. I think I know why this is, it's reduced base current through the o/p transistors causing the transformer primary impedance to increase. Open-loop gain goes up, and we get into phase-shift 180° and gain >1 territory, and off it goes. It occurs at HF because of normal transformer phase-delay at HF. It's not a fatal flaw (as in, nothing catches fire or anything like that), but it's just annoying that it happens and it's 'not correct'.

I thought I had Zobel networks across the speaker terminals to try to address this, but upon taking the top cover off today, there's nothing there. Obviously they didn't do the job then, so I took them out. The speaker terminals are not the best location for them anyway.


Newly modified schematic below, so far! Surgery may commence imminently. Or not ...
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Germanium amp 2024-C.png
Germanium amp 2024-C.png (110.13 KiB) Viewed 515 times
 
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andrew Ivimey
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#146 Re: Germanium Amplifier

Post by andrew Ivimey »

PNP oc35s ( never heard of them) I always thought oc28s were the swanky ones!

I'd like to hear them.
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#147 Re: Germanium Amplifier

Post by simon »

This is cobblers and do this better is my kind of self-encouragement!
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Ray P
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#148 Re: Germanium Amplifier

Post by Ray P »

Warmer weather is on the way Mike, soon be working on your geranium amplifier...
Sorry, I couldn't resist!
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Dave the bass
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#149 Re: Germanium Amplifier

Post by Dave the bass »

I was part-paid (aka "take them please before I dump them on eBay") for a repair job a while back in a few OC35s! Matched pairs apparently back in the 60s or 70s and stored in these polystyrene boxes.

Shiny shiny!
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andrew Ivimey
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#150 Re: Germanium Amplifier

Post by andrew Ivimey »

Wowser, they make 2N3055s look serious!
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