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Old 27th July 2004, 02:50 AM   #141
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by Sheldon
So if both channels were fed with one resistor/diode pair and 500 ohm pots (which would be set near the middle), I'd get about 500 ohms from one channel to the other. To eliminate crosstalk, I'd guess would require either higher value pots or completely separate circuits for each channel (best, I'm sure, but hey it's worth a try to use fewer parts)?
Donít feed the chipís input directly, use the resistor of a few tens, maybe 100 kOhm. Donít forget that the diodes will be low impedance points.

Quote:
The RC filter would be a low pass filter set to pass anything higher than 10hz or so to ground?
Its cut frequency can not be too low, but HF properties of bigger caps can be too badÖ

Quote:
Originally posted by sek
How does the LM3875 start up when connected to an unregulated supply?
Thus you donít know it does that.

Quote:
Originally posted by sek
So, does a turn-on noise only occur due to the fact that the input buffer powers up slowly and experiences offset "noise" at turn-on? In other words, can I use the LM3875 (without muting circuit) safely with a regulator supply?

My circuit doesn't contain a buffer, one can assume that the "source" circuit is turned on and stable once the GC is powered up.
Thump is solely buffer related problem. Talking about solid state buffers, at least discrete buffers I tried in GC, the problems finish in about or in less than a half of the second.

Pedja
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Old 27th July 2004, 05:11 AM   #142
Sheldon is offline Sheldon  United States
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Quote:
Originally posted by Pedja

Don?t feed the chip?s input directly, use the resistor of a few tens, maybe 100 kOhm. Don?t forget that the diodes will be low impedance points.




Thump is solely buffer related problem. Talking about solid state buffers, at least discrete buffers I tried in GC, the problems finish in about or in less than a half of the second.

Pedja
Thanks Pedja, I think I might give it a try. I can use a bit larger cap and bypass it with a smaller one (sounds like I know what I'm talking about here, but don't be fooled - I'm just guessing).

As for the buffer thump, I found something that may be interesting. As you might remember, I rigged the selector switch so that one position shorts the buffer output to ground. In this position, when I turn on the amp I get the very slightest click on output. No noise at all when I then switch the buffer to the "normal" position. This with a ss cd player. However, when I use my tube based pre for the source, I get a fairly loud pop if I switch between "short" and "normal" for the 30 seconds or so when the pre is warming up or shutting down. No pop when the pre is fully on or off.

Sheldon
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Old 27th July 2004, 08:55 AM   #143
Nuuk is offline Nuuk  United Kingdom
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Quote:
However, when I use my tube based pre for the source, I get a fairly loud pop if I switch between "short" and "normal" for the 30 seconds or so when the pre is warming up or shutting down.
Be careful Sheldon. My findings with the valve buffer indicate that you may get more than an irritating noise if you connect the amp to the buffer while it warms up! More details here .
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Old 27th July 2004, 03:10 PM   #144
Sheldon is offline Sheldon  United States
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Thanks Nuuk,

Normally I am careful about that. I discovered this one accidentally. Lulled into complacency by the fact that the pre-amp is very well behaved. Even if I turn it on or off with the usual power amps on (not recommended, I know), I get no noise at all. We are sometimes a bit spoiled by all the fail-safe features added to most commercial gear. Many here believe that some of those features have a sound quality cost, and should be left out. May be true, but then we have to remember to be a bit more careful.

Sheldon
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Old 30th July 2004, 09:29 PM   #145
Sheldon is offline Sheldon  United States
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Default Re: Dc Offset

Quote:
Originally posted by Sheldon
If I understand Malichi's post correctly, the circuit could go something like this:

10k resistor and diode to ground on each side of the p.s. (+/-25V). With a schottky diode, the voltage between the two leads, taken between resistorand diode, would be about 800mv. To get resolution to about 5 mv would require 0.5% precision. If that's not practical with a 1 turn pot, could an additional divider be used between the pot and diode? Would the resistor/diode approach sufficiently eliminate the noise issues so that an RC filter would not be required for the offset circuit? If the pot is a high enough value (say 50k) could pots for both channels use one set of resistor/diodes?

Sheldon
After getting some good suggestions from Malichi and Pedja, I gave it a try. The +25v feed went to a pair of 11k resistors in series, followed by a diode to ground and to one side of a 500ohm multiturn pot. Between the 11k resistors, I connected a 3uf electrolytic and a 0.1uf film capacitor, in parallel, to ground. The other side of the pot was connected to a duplicate circuit for the -25v (with electrolytic and diode in reversed orientation, of course). The center lead of the pot went, via a 90k resistor, to the signal in for the chip amp board.

I was able get appropriate voltage output from the trimmer circuit (very low mv) , but it was not very stable when all hooked up. After taking some measurments with my digital multimeter, I'm guessing (Pedja?) the low impedence of the discreete regulator created a current path between sections of my divider circuit. Sooo--, since my original DC offset was negative in both channels, I completely disconnected the negative half of the divider and connected that side of the trim pot to ground instead. That way I could inject from zero to positive, to trim the neg. offset. Works fine (keeping in mind that this is all probably unnecessary overkill, as I couldn't hear any bad effects from even 100mv of DC offset from the amp). I can trim to within a millivolt or two at the amp output, and the total offset seems as stable as without the trim circuit. No loss of sound quality, as far as I can tell. No channel cross talk. In fact, this little amp sounds pretty darn good, and runs quite cool. If I push it hard, the regulator output transistor sinks get hot to the touch, but that's it. I might improve the sinking if I want to drive something other than my 90db efficient z-Horns, but with those, no problem. Actually, I've used it on some bigger 87db three way speakers (Raven ribbon, 4" and 8" Seas Excell) and it sounds very nice. Drives them no problem too, but might need more sinking for the reg. transistors for heavy duty on those.

I checked out this DIY site to learn something about speakers. Ended up doing a complete amp with some discreete circuits, and even this little extra experiment. Thanks to Brian GT for the initial inspiration (made it easy with his board and component group buy), PD for a chassis that would go on megabuck commercial gear, Pedja for circuits and explanations so clear, that a complete novice (me) was willing to take a shot, and all the others to give freely to the DIY community.

Sheldon
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Old 30th July 2004, 10:32 PM   #146
SRMcGee is online now SRMcGee  United States
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Default Amazing

Sheldon:

Congratulations on your efforts! Is this project now in a state where you can share your end result with others? I am in the process of constructing four of BrianGT's GainClones and would be very interested in joining in on a group buy to add active buffer input circuits to the passive preamp portions of my project.

Regards,
Scott
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Old 31st July 2004, 03:38 PM   #147
Sheldon is offline Sheldon  United States
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Default Re: Amazing

Quote:
Originally posted by SRMcGee
Sheldon:

Is this project now in a state where you can share your end result with others? I am in the process of constructing four of BrianGT's GainClones and would be very interested in joining in on a group buy to add active buffer input circuits to the passive preamp portions of my project.

Regards,
Scott
I'd be happy to share any specific info, or elaborate on my posts here. If CJD does come up with some boards, I'd join in.

Sheldon
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Old 1st August 2004, 12:53 AM   #148
cjd is offline cjd  United States
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ergh.

I've been working excessively of late (better than not working, mostly) so I'm barely keeping up with the conversation at the moment. If that - I think I need to re-read stuff because I've only partially absorbed it.

Hope y'all aren't impatient.

C
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Old 1st August 2004, 04:41 AM   #149
Sheldon is offline Sheldon  United States
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Originally posted by cjd
ergh.

Hope y'all aren't impatient.

C
Please don't read that into any of my replies. Given what we are paying for the design service, we can hardly be demanding.

Sheldon
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Old 3rd August 2004, 01:24 AM   #150
cjd is offline cjd  United States
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heh. I wasn't suggesting anyone was being impatient. Except my wife, who is using up most of my free time having me paint and install hardwood (bamboo actually) floor.

The good news is, it looks like a matching set of these boards will all sidle up to each-other on a single PCB quite nicely. That's power, both amp boards, both buffers.

C
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