Cheap as Chips

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Nick
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#16 Re: Cheap as Chips

Post by Nick »

Mike H wrote:
Paul Barker wrote:shows a sink working at 2 mA no chance of that with a dn2540.

Think you need to be 10mA and above. But definitely not down in single figures.
Why?
Because the simple depletion mosfet CCS uses the resistance in the source to generate the voltage to shut off the current, The higher the voltage (more negative) on the gate, the lower the current. Even if the device was linear and a doubling of the gate voltage meant a halving of the current, to double the gate voltage with half the current through the source resistor you would need four times the resistance, so the relationship between current and resistance is exponential, so eventually you can't increase the resistance enough to reduce the current so there is an effective lower limit to the current you can regulate.

Page 4 of this provides a way of producing a precise current source with a depletion mosfet that will go down to any current you want.

http://www.ixys.com/documents/appnotes/ixan0063.pdf

Not tried it yet though, must try it some time.
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#17 Re: Cheap as Chips

Post by Paul Barker »

Thanks.
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#18 Re: Cheap as Chips

Post by IslandPink »

little eddy wrote:I may be a little side tracked but got me thinking about my main amp with a single 10m45 running at 20mA .
Is the dn2540 preferred to the 10m45 ?
I have a few of the latter. Is the consensus that the improved cascode design is worth implementing and assume the same type of circuit would benefit the 10m45?

What sort of audible benefits might this bring about?
Are we talking as CCS for anode load here ? - in which case I can offer opinions ....
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#19 Re: Cheap as Chips

Post by little eddy »

IslandPink wrote:Are we talking as CCS for anode load here ? - in which case I can offer opinions ....
Yes
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#20 Re: Cheap as Chips

Post by pre65 »

I use an anode CCS on the 12GN7 driver valve for the GK71 monoblocks.

It's set for about 30ma (from memory.

Originally it was one 10M45s on a big heatsink, but now its cascaded with two DN2540n5. Can't remember if it made much difference.
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#21 Re: Cheap as Chips

Post by IslandPink »

Definitely try a cascode - DN2540 ideal.
Less capacitance 'visible' to the valve. Improved tone and less 'SS' sound.
See this , small table at the end summarises the performance -
http://www.pimmlabs.com/web/ccs_performance.htm
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#22 Re: Cheap as Chips

Post by jack »

Nick wrote:Page 4 of this provides a way of producing a precise current source with a depletion mosfet that will go down to any current you want.

http://www.ixys.com/documents/appnotes/ixan0063.pdf

Not tried it yet though, must try it some time.
I came across that AN whilst doing some other research and its an interesting document with some novel designs, however my concern with using the Fig 3 (page 4) one is that all these designs are for DC/industrial use - AC, specifically audio, response is paramount for us. The scheme is certainly worth a try - like you, I just haven't got round to it yet.

The "Improved Jung" CCS is interesting - it's input rejection is better that the standard cascode variant which is in common use (same as in the MJSR schematics etc.) but we're splitting hairs here - whilst there is a 10dB improvement, we're talking the difference between 150dB and 160dB rejection at low frequencies - I would challenge anyone to distinguish between those two...

What IS relevant is that these CCSs have been tested for their AC response - I would give either or both a go and see what you like.
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#23 Re: Cheap as Chips

Post by little eddy »

jack wrote:What IS relevant is that these CCSs have been tested for their AC response - I would give either or both a go and see what you like.
Would the 'augmented cascode circuit style work with the 10M45s as I have a few of them?

Also what are the relative thermal load of the two transistors - I assume that the power is not equally split across the two.
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#24 Re: Cheap as Chips

Post by little eddy »

Well the bits have arrived. Maybe this weekend can start some soldering of the 3886 boards and testing of the pre-amp board.

Should have been wrapped in old news paper!
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#25 Re: Cheap as Chips

Post by jack »

little eddy wrote:
jack wrote:What IS relevant is that these CCSs have been tested for their AC response - I would give either or both a go and see what you like.
Would the 'augmented cascode circuit style work with the 10M45s as I have a few of them?
No - these are completely different - they are actually an n-channel enhancement mode FET with an error amplifier and are only good for 10mA - see the data sheet at http://ixdev.ixys.com/DataSheet/98703.pdf
little eddy wrote:Also what are the relative thermal load of the two transistors - I assume that the power is not equally split across the two.
The first one dissipates all the power - the second one only has a few volts across it and the TO-92 variant would probably do just fine.
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#26 Re: Cheap as Chips

Post by little eddy »

So the operating point of the valves is Va 42V, Vg 0.6V and Ia 3mA. B+ is 56V.

Have fitted the CV4010, and tested/installed the J112 CCSs @ 3mA.

There are 1uF dc decoupling caps on both the inputs and outputs. The F-3dB point on the input calculates fine - 470k grid leak gives an F-3dB of 0.34Hz, (MJ recommends 2Hz).

But the output is grounded by a 12k resistor and to me this gives a F-3dB of 13Hz, so I think I need to increase either the resistor, capacitor or both.

I don't expect the decoupling caps are particularly good and guess these have one of the greatest influences on the sound quality. So could consider something like Wima MKS4 4.7uF 100V. I could just increase the output resistor to 100k but would the EF95 prefer the output resistance low like the 12k? Any particular views/recommendations? The board could take capacitors up to around 15mm diameter and 30mm long.
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#27 Re: Cheap as Chips

Post by little eddy »

Slow work but got my mods made to the pre-amp section so hopefully will be able to give it a listen to soon.

The input and output caps are 1uF'Audiophiler' ones from China because I seen a few of these used on the 3886 power cards (and were cheap).

Ended up increasing the Rout to 100k as the power amp looks like it has an input resistance of 23k so didn't want it below 10k, (can't believe they fitted the pre-amp with only 12k ones). I will fit 4.7uF 'Audiophiler' input caps to the power boards to have a decent F-3db. I had though of removing one of the CR circuits but just thought it best to keep them both as stand-alone boards.

I'm using the carcase of a little portable hi-fi, the pre-amp bolted underneath the top section. Hopefully should look quite neat when finished.

Next up is are the power boards and heatsinks that I am mounting on the two sides - just need to get myself some metric taps - when I checked my 'apprentice' tool box, the ones I had were BA so shows how long since I used these!
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#28 Re: Cheap as Chips

Post by little eddy »

Well all was progressing fine until:

I built-up the preamp, and tested it with an old SS amp in the garage and all sounded promising.

Next up was the power amp. Built and powered up - no noise. Good or bad?

So wired in the pre-amp and hum, hum, hum. Not horrendous but quite annoying and could not live with.

Played around with all of the flying leads but no change.

I had no earth connection so put an earth onto the CT of the power board power supply. No change.

What I did notice was the gain of the LM3886 boards is probably significantly higher than I envisaged because just one notch on the volume control and there was reasonably loud music, and it didn't sound bad - and you couldn't really hear the hum so perhaps slightly promising.

Disconnected the old CD player and just hum again.

So could:
1. try a more structure approach to the earthing. On the pre, there is continuity between the 0V terminal and both the input and output gnd terminals. And then on the power board looks like there is continuity between the DC supply centre tap, and both input and speaker 0V terminals. With there being effective gnd connection between the two via the pre to power signal cable, though that separate earth leads to both pre and power boards would have a greater chance of an earth loop. Will try putting gnd to pre 0V and power CT terminal as per their individual drawings but this will give me a 'ring' and can't see how this will help.
2. Add an earth lift circuit
3. Maybe a cop out but with significantly more gain than needed, put a voltage divider on the pre-amp output because I need to do something just to give some degree of volume control anyway.
4. Or put the voltage divider on the pre-amp input to limit to less than 0.6V peak, (Vg of the pre-amp), and then put the volume pot on the pre-amp output. This perhaps sounds like the cleanest if I can't resolve the hum. But I need to consider that my daughter may drive this from her phone and until she gets this as a present, can't really try to see what the peak output would be from her headphone jack.

Any other ideas if these fail?
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#29 Re: Cheap as Chips

Post by Mike H »

I've lost track, does the valve pre-amp have any Voltage gain, or is it just a cathode follower?

Also the LM3886 may have too much gain and the earthing for it could be quite fussy.
 
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#30 Re: Cheap as Chips

Post by little eddy »

Thanks Mike.

Yes the EF95 in triode mode does have gain and I guess my use of a CCS doesn't help.

But the gain of the LM3886 is significantly higher than the old SS amp I have in the garage. There is so much gain I can put quite a big voltage divider between pre and power and still get sufficient volume for our domestic use. This also reduces the hum as I increase the series resistance so points to this hum coming from the pre-amp.

Will still try separate earths to pre and power boards and also maybe only to the pre, and also an earth lift circuit.
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