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Old 6th November 2012, 10:44 PM   #91
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Default Why not simply dial it in?

The solo chip amp is highly sensitive to current, especially at feedback, so here's some dials for you.
Note: Variable resistors made from multi-turn trimpots--solder 2 out of 3 pins together before use, and then you have a 2-pin variable resistor--check with ohmmeter before use.

Setup:
(LM1875 non-inverting amp--see the datasheet)
♦ The input load resistor is a simple 10k.
♦ The input cap is 2.2u (like Nichicon ES 2.2u and 4.7n polyester bypass)
♦ The NFB cap is 470u (consider 1u or smaller electrolytic for bypass, or a 22n or smaller polyester dip cap for bypass, but don't use polypro).
♦ The 2 amp board power caps are also 470u, and they can be exactly the same model and same bypass efforts that you find attractive for NFB cap.
♦ The feedback resistor is 10k series to 250k and Parallel that 250k with a 100K multi turn trimmer (creating a 72k variable resistor)--total Range of variance is then 10k to 82k.
♦ The feedback-shunt resistor is a 1k series to 2.2k and Parallel that 2.2k with a 10k multi turn trimmer (creating a 1.8k variable resistor)--total Range of variance is then 1k to 2.8k.

Adjust:
Although the trimmers have been restricted to decent values, please be careful to avoid setting the gain outside the range of 10x to 40x. You'll need to use your ohmmeter between settings. Divide the feedback and feedback-shunt and then add 1 to find the gain setting. These two dials vary both gain and feedback current, and thus a vast variety of differences is possible, and the choice is yours.

Reason:
Turning a dial could be easier than hours of soldering different resistor values. After dialing in optimal resistor values, we could then replace trimmer arrangement with nearest value ordinary resistor instead.

Alternatives:
Dialing in feedback settings is not the only way to fine tune LM1875. External compensations can be added for that job. That is doable, but I couldn't possibly explain it. In this case textbooks can be really helpful. Much textbook material for small signal op-amps can be applied to LM1875 since it is a simple op-amp.
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Old 6th November 2012, 11:04 PM   #92
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Quote:
Originally Posted by danielwritesbac View Post
The solo chip amp is highly sensitive to current, especially at feedback, so here's some dials for you.
Note: Variable resistors made from multi-turn trimpots--solder 2 out of 3 pins together before use, and then you have a 2-pin variable resistor--check with ohmmeter before use.

Setup:
(LM1875 non-inverting amp--see the datasheet)
♦ The input load resistor is a simple 10k.
♦ The input cap is 2.2u (like Nichicon ES 2.2u and 4.7n polyester bypass)
♦ The NFB cap is 470u (consider 1u or smaller electrolytic for bypass, or a 22n or smaller polyester dip cap for bypass, but don't use polypro).
♦ The 2 amp board power caps are also 470u, and they can be exactly the same model and same bypass efforts that you find attractive for NFB cap.
♦ The feedback resistor is 10k series to 250k and Parallel that 250k with a 100K multi turn trimmer (creating a 72k variable resistor)--total Range of variance is then 10k to 82k.
♦ The feedback-shunt resistor is a 1k series to 2.2k and Parallel that 2.2k with a 10k multi turn trimmer (creating a 1.8k variable resistor)--total Range of variance is then 1k to 2.8k.

Adjust:
Although the trimmers have been restricted to decent values, please be careful to avoid setting the gain outside the range of 10x to 40x. You'll need to use your ohmmeter between settings. Divide the feedback and feedback-shunt and then add 1 to find the gain setting. These two dials vary both gain and feedback current, and thus a vast variety of differences is possible, and the choice is yours.

Reason:
Turning a dial could be easier than hours of soldering different resistor values. After dialing in optimal resistor values, we could then replace trimmer arrangement with nearest value ordinary resistor instead.
I've seen dip switches on paralleled resistors to achieve the same. 16 available values on a 4 line dip switch/4 resistor. Very easy to then replace switch with appropriate jumpers when good values are found.
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Old 7th November 2012, 02:31 AM   #93
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Quote:
Originally Posted by Robert Kesh View Post
I've seen dip switches on paralleled resistors to achieve the same. 16 available values on a 4 line dip switch/4 resistor. Very easy to then replace switch with appropriate jumpers when good values are found.
Thanks! That is really great! Got an example? Maybe a schematic?
Quote:
Originally Posted by danielwritesbac View Post
♦ The 2 amp board power caps are also 470u, and they can be exactly the same model and same bypass efforts that you find attractive for NFB cap.
That was badly phrased. My apologies! I meant to say that the "power caps of the amplifier board" could be the same 470u value and same high quality as the cap you've selected for the NFB cap.
All of the caps that are located on the amplifier board are important.
If there's no other means to test the quality of caps, you can install a sample as your NFB cap and literally eavesdrop on the quality.
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Last edited by danielwritesbac; 7th November 2012 at 02:34 AM.
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Old 7th November 2012, 02:35 AM   #94
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by danielwritesbac View Post
The sound of an expensive discrete amp? That is the sound of current headroom. Parallel chip amplifiers can do it too.
Everything is just relative. Good or bad, that's relative. I have been searching for the best amp for so many years. Isn't it very silly if everything is actually available in a cheap LM1875? I must be deaf or having a hallucination...

LM1875 is not a new toy for me. I don't expect it to sound like a high end amp. It is not even the chip I have achieved the best system with.

But the real problem with my current implementation is probably the input opamp. I have achieved several outstanding chip amp systems, and all are with pure class-A front ends. I choose opamp here so it can become a "benchmark reference". May be I should use B1, or DOZ preamp?

My current opamp input stage uses dual-opamp socket (2xopamps in one package). I don't have enough inventory for this. Last night I tried several opamps form 0.3 to 15V/us, and I have to accept that (in that circuit) the best sound was given by an Analog Device opamp with 1.8V/us.

While Sanyo Oscon on the feedback path gave cleaner sound, the bass sounds "colder" and I don't like.
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Old 7th November 2012, 06:27 AM   #95
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Quote:
Originally Posted by Jay View Post
Isn't it very silly if everything is actually available in a cheap LM1875?
Just almost. LM1875 does have a failing, but the workaround isn't difficult. The soundfield is as deep as a hi-fi should do, but curiously not wide. The fix is to build monoblocs for their greater stereo separation. Also, if you desire a more natural tone or even a dark voice tone, the parallel LM1875 can do that.
Well, they're quite good and indeed the quick trip to hi-fi; however. . .
Actually, my favorite amplifiers are the wild dynamics Circlophone and also my old TDA7294 (with rather excessive fine tuning).
Quote:
Originally Posted by Jay View Post
But the real problem with my current implementation is probably the input opamp. I have achieved several outstanding chip amp systems, and all are with pure class-A front ends. I choose opamp here so it can become a "benchmark reference". May be I should use B1, or DOZ preamp?
You've already got an op-amp. Why stack another for quality reduction? The LM1875's gain capacity goes high enough to clip itself even if driven by an MP3 player. Sufficient gain capacity is one of the conveniences of the 25 watt amplifier, and there's no need of a preamp to increase the clipping.

However, if you've got the unavoidable urge to stack another op-amp in front of the power op-amp, try nesting/composite topology to negate insertion loss. Instead of reducing quality, nesting/composite topology can make improvements. ah! That also includes more control over the sound of your power op-amp.

P.S.
Other answer: DOZ is close. I like the very similar MooseFet triode simulator preamp since it is very clear and so easily adaptable to suit a vast variety of personal preferences.
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Old 7th November 2012, 08:10 AM   #96
Jay is offline Jay  Indonesia
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Quote:
Originally Posted by danielwritesbac View Post
Well, they're quite good and indeed the quick trip to hi-fi; however. . .
Actually, my favorite amplifiers are the wild dynamics Circlophone and also my old TDA7294 (with rather excessive fine tuning).
Yeah, a cheap trick. My resistor is even more expensive than the chips

There has been many blind tests around since Gainclone popularity some years ago. The winners have been always the overture series. I wish I could make a TDA7294 to sound good. Because I like the mosfet sound of it.

Quote:
Originally Posted by danielwritesbac View Post
Also, if you desire a more natural tone or even a dark voice tone, the parallel LM1875 can do that.
I just did that again last night, but I never perceived/noticed special changes to the voice tone. It is just more dynamic.

Quote:
Originally Posted by danielwritesbac View Post
You've already got an op-amp. Why stack another for quality reduction?
I thought chip amp guys might like tone control, no?

Quote:
Originally Posted by danielwritesbac View Post
Sufficient gain capacity is one of the conveniences of the 25 watt amplifier, and there's no need of a preamp to increase the clipping.
No I don't think a chip by itself, especially small ones, can sound good enough for me without a driving preamp (low gain or buffer). Especially considering the use of potentiometer for volume control.

Quote:
Originally Posted by danielwritesbac View Post
However, if you've got the unavoidable urge to stack another op-amp in front of the power op-amp,
I have an (useless) idea to stack at least 10 very low noise opamps (OPA111) in front of a very high slew rate opamp (at least 1000V/us) for a preamp. May be I can put them in front of another power opamp

Quote:
Originally Posted by danielwritesbac View Post
Other answer: DOZ is close. I like the very similar MooseFet triode simulator preamp since it is very clear and so easily adaptable to suit a vast variety of personal preferences.
I have experienced twice a good system using chip amps. It is of course very dependent on other components such as the speaker.

First one, is a TEA chip, no more than 2W, driven by a preamp similar to the MooseFet (but better of course). At that time, I have tube amp and several Alephs (So you know that the benchmark is not that "low").

Second one, TDA2009 chip, no more than 3W for 0.1% THD, driven by a preamp similar to DOZ.

Another one was not really a "success", LM3875 with triode preamp, with power supply full of chokes. Inspired by JLTi I guess.

But chip amps are just chip amps. I do this just for fun because I have the parts scattered all around the house.
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Old 7th November 2012, 09:53 AM   #97
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Jay View Post
But the real problem with my current implementation is probably the input opamp. I have achieved several outstanding chip amp systems, and all are with pure class-A front ends. I choose opamp here so it can become a "benchmark reference". May be I should use B1, or DOZ preamp?

My current opamp input stage uses dual-opamp socket (2xopamps in one package). I don't have enough inventory for this. Last night I tried several opamps form 0.3 to 15V/us, and I have to accept that (in that circuit) the best sound was given by an Analog Device opamp with 1.8V/us.

No I don't think a chip by itself, especially small ones, can sound good enough for me without a driving preamp (low gain or buffer). Especially considering the use of potentiometer for volume control.
the chipamps do not need a buffer.
The non-inverting topology allows virtually any Zin value to be adopted and there is absolutely no value in increasing this further.

It's the source that may need a buffer.

The source must be capable of driving the cable and the Rin that follows. That Rin can and should include RF filtering and should also include a DC blocking capacitor.

If you have a vol pot driving a cable expect there to be problems. vol pots cannot properly drive cables.

Sort your source and remove the buffers from the front end of the chipamp.

There is one exception. Inverting topology Chipamps. These can have a lowish Rin. Few sources can drive these directly. Here a "converter" is a real performance benefit. See Cordell's jFET LTP "converter".
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Old 7th November 2012, 11:12 AM   #98
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Quote:
Originally Posted by Jay View Post
I wish I could make a TDA7294 to sound good.
Well, do it.

TDA7294 non-inverting amp:
Pair 220u amplifier board power caps and a single 2u polyester rail to rail.
47u (or 68u) for bootstrap
10k for input load resistor
10k for mute (no cap)
22k with 10u timer for standby
60k (120k||120k) for feedback resistor
2.7k for feedback-shunt resistor
220u (or more) for NFB cap
1u (or less!) for input
Output zobel is 10n with ~8R
Add RF filtering to input cap
Use a stiff power supply (20,000u per rail, 3a per chip)
Use less voltage than max (under-volt)
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Old 7th November 2012, 11:25 AM   #99
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Nobody mentionned LME498XX chips + discrete outputs.
A choice i prefer when some power is required compared to paralleling plain chipamps.
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Old 7th November 2012, 11:52 AM   #100
AndrewT is offline AndrewT  Scotland
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That proposal is mentioned many times. Maybe it is just not in this Thread.
I never recommend paralleling and/or bridging of chipamps.
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