Germanium Amplifier

For the three and more legged things
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Mike H
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Post by Mike H » Sun Oct 21, 2012 9:48 pm

It's making noises!!


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pre65
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Post by pre65 » Sun Oct 21, 2012 9:49 pm

Mike H wrote:It's making noises!!


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Wahey. :)
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Post by colin.hepburn » Sun Oct 21, 2012 10:01 pm

Been watching mike
Are the noises its making good ones? :) 8)
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Post by Mike H » Sun Oct 21, 2012 10:11 pm

Not shabby at all actually. :thumbleft:

Very interesting treble .... can't remember hearing owt quite like it
 
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Post by Mike H » Sun Oct 21, 2012 10:18 pm

Revised circuit diagram ~ (large picture hence link)

http://livinginthepast-audioweb.co.uk/i ... 2012-3.gif

The power supply ~ (ditto)

http://livinginthepast-audioweb.co.uk/i ... mp-psu.gif
 
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Post by Mike H » Sun Oct 21, 2012 10:42 pm

Mods to amplifier schematic ~

First tested PSU with the bench supply, OK, then wired to transformer with o/p stage connected only, OK.

Then when I first fired it up with everything connected (i.e. plus input/driver) nothing caught fire, but not long probing with DMM I got that impression like you do that summink's a bit awry somewhere. Well you do if you've experienced RF oscillation in a valve amp, you get weird sort of DC readings. It was a bit like that.

Put DMM on AC onto speaker terminals, sure enough there was 2 - 3V of ~ something ~ coming out.

As it happens the Fluke has frequency counter function, so switched to that and ~ it's oscillating at 150 kHz! (-ish.) (It's interesting in itself that the Fluke can not only measure that sort of frequency, but the AC Voltage of it.)

Adding C7 stopped that. Obviously it's because bandwidth of the transformer doesn't go that high, so it doesn't get through to the o/p stage and so C6 can't do anything about it.

#2. The MOSFET current is a mite too high @ 40 - 45 mA whereas I preferred 30 - 35 mA. This is interesting 'cause bonkers MOSFET amp does that in the same set-up, but there you go, gate Voltage must be lower on these ones.

This was addressed by increasing R11 to 10k, and decreasing R8 to 4k.

As an aside, the DC difference across R14 is only about half a Volt, in case you were wondering.

Aside #2 ~ it may have escaped anyone else's notice that the open-loop gain on this is not great. This is deliberate so that the damping factor is on the low side, like a valve amp with little or no NFB (note also local feedback is applied between input & driver by R6, which increases the effect). It was proved in simulation anyway that it does have this sort of behaviour.

Despite the o/p coupling cap being 'only' 1,000 uF, the bass is all there, again much as though the damping factor is low. Like a valve amp.


Still working! Got a Radio 2 jazz prog on at the mo.
 
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Post by Mike H » Sun Oct 21, 2012 11:15 pm

Now the David Jacobs show. Very similar.


More photos ~

Power supply board ~

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Finished amp ~ (sans lid)

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Post by Mike H » Mon Oct 22, 2012 1:37 pm

Still working! :D

Modifications!

Modified diagram:
http://livinginthepast-audioweb.co.uk/i ... 2012-4.gif


#1. Phil has a saying "transistor ear", I was getting a bit of this last night. The treble is quite 'hard'.

Mind you it's wonderfully detailed, sort of like the music is put under a microscope so that nothing escapes. Janice Long played The Jam 'Down In The Tubestation At Midnight' and there is this subtle percussion effect going on on the right channel for the choruses, guess it's supposed to be like clickety-clack of wheels on rails, never knew that was in there before.

Later, Kate Bush 'Babooshka', I should know this quite well, there's a 'yelling' bit at one point in the background which I should already know exists but not heard it as clearly as this before, it was quite surprising.

Notwithstanding, it's still a bit overwhelming.....

#2. The bass roll-off is a bit excessive (reactance of C2), so there's a perceptible lack of 'body' at the bottom end of the midrange.

#3. Too much gain! Vol controls were only 10 - 15 degrees off the bottom and it was already a bit too loud.

To get around these R18 & C8 were added.

It sounds a bit 'warmer' now.
 
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Mike H
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Post by Mike H » Mon Oct 22, 2012 3:37 pm

Image


Come here there's more .....


Still on the trail of the bright treble, as a sperrymint I tried inserting 20 uH RF chokes (I just happened to have found) in series with the speaker 'live' terminals. That toned it down a bit! The tweeters are now restored to their 'normal' level (on 50R WW pots), previously I had to turn them down! Still didn't help much....

I've had to to this before on amps using global NFB, an RF choke on the output makes them much more stable. Maybe it doesn't like the speakers leads capacitance, or something like that. That's the usual reason.

The RF choke isolates the o/p point where the NFB is taken from, from the external load and gives it a better chance to feedback some actual HF.

20 uH is only 1 Ohm @ 8 kHz, so as a pure reactance in series with a speaker that would be inaudible, but that isn't what I'm hearing, it's made a big difference. Much more mellow.

In a relative context I hasten to add, it's not really a 'mellow' amplifier but the treble is much more under control and consequently the midrange sound better as a result.
 
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Mike H
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Post by Mike H » Mon Oct 22, 2012 10:02 pm

Forget everything I've just written about RF chokes, it's all cobblers. Must be something to do with FM .....

Now onto CD's and everything's back to as previously, including tweeters wound up to normal ...... Image
 
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colin.hepburn
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Post by colin.hepburn » Tue Oct 23, 2012 1:17 am

Mike H wrote:Forget everything I've just written about RF chokes, it's all cobblers. Must be something to do with FM .....

Now onto CD's and everything's back to as previously, including tweeters wound up to normal ...... Image
Yes I use two or three high Quality tracks that I know well on two CDs and two tracks on a DVD Diana krall live in Paris all running through the DAC

Why the DVD well how many times have you watched a DVD just through the TV speakers perhaps and you can’t hear the instrument what the band member is playing The DVD is great for this picking out the micro detail when you have a visual on it that could be misted on a CD :)
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Mike H
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Post by Mike H » Tue Oct 23, 2012 3:12 pm

Yes exactly. :thumbleft:

Not having a 'proper' TV at the mo, I watch everyfink off the PC with the audio adaptor so it all goes through the stereo anyway so know what you mean.


Just to double-check there isn't something odd going on with the FM tuner, this morning I got it going in parallel with the Troughline so I could compare, they're exactly the same so it's all to do with FM quality. Which as we know is heavily modified with compression / treble boost / normalization / noise gating et al ~ and which is what makes Classic FM sound so dreadful at times.

Having said that, Radio 2 seems less abrasive today so, as the amp has been working a couple of days now, maybe the caps have settled down or something ~ I try to avoid using expressions like 'burn in' :D but could be something like that.

Mind you doesn't help it being so sensitive, so I have resorted to adding a 100k resistor to each pot to reduce the level, otherwise it's quite difficult setting and balancing a listening level so close to the bottom of the tracks. (The resistors are SMD thick film which 'I can't hear' :D , just in case of altering anything.)


Problee safe to say this is the best SS amp I've ever made, mind you haven't done anything like this for what must be over 30 years. Sure better than anything I could have turned out back then, ignerunt as I was in them days :D


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Image Image
 
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Mike H
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Post by Mike H » Tue Oct 23, 2012 3:26 pm

Just one remaining minor fly in the ointment ~ well a big one actually ~ the mains transformer BUZZES LIKE CRAZY Image ~ it's doing my head in.

So I've been onto my transformer man for something more betterer in the concentric wound line, (original is split-bobbin ~ ZZZZZ Image ), first thing he reckons is the stack should half an inch thicker for a start.


Image
 
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pre65
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Post by pre65 » Tue Oct 23, 2012 3:36 pm

Well done Mike. :)

Yes, buzzing transformers are a pain, got one in my "best" DAC. :(
People are like stained-glass windows. They sparkle and shine when the sun is out, but when the darkness sets in, their true beauty is revealed only if there is a light from within
Dr Elisabeth Kubler-Ross

G-Popz THE easy listening connoisseur. (Philip)

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Mike H
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Post by Mike H » Tue Oct 23, 2012 3:41 pm

Ta Phil Image
 
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