Germanium Amplifier

For the three and more legged things
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thomas
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Post by thomas » Thu Nov 29, 2012 6:06 pm

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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Post by colin.hepburn » Thu Nov 29, 2012 7:49 pm

Great stuff mike nothing wrong with a good SS designed Amp 8) :)
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Mike H
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Post by Mike H » Fri Nov 30, 2012 2:59 pm

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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Post by Mike H » Sat Dec 01, 2012 4:12 pm

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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Post by Orit » Tue Dec 18, 2012 12:55 pm

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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Post by Mike H » Tue Dec 18, 2012 4:54 pm

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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Mike H
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Post by Mike H » Mon Jun 17, 2013 12:09 pm

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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Post by Dave the bass » Mon Jun 17, 2013 2:42 pm

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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Mike H
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Post by Mike H » Mon Jun 17, 2013 3:37 pm

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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