Might have made an oops

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Jeff:

The boards came in the other day Here are photos of the top and bottom. I'm totally tied up until August but will build one for myself first and then we can set up a time to get together.

Tom

Attachments

[tomp]

Here is the first board I assembled for myself. I just finished it and have not had time to test it yet. I won't get to test it until the first week of August. There are a few locations that are not populated. C1 and C201 are locations for whatever capacitance the cartridge needs for proper response. R11 and R211 are only used if the input is capacitively coupled. It will then provide a path for the input bias current of the line stage OP Amp to ground.

Tom

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[JDCrae]

That looks amazing Tom! Cannot wait to go over things in person to understand what is going on. As far as the turntable, I know nothing about it. Would it be helpful to take a picture of it?

I am free in August except for Aug 6th, and the 12th-19th. Just let me know what works for you.

Thank you again for all of this.

Jeff

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Jeff:

I had some time to test the board today. I'm enclosing a few scope traces that show square wave performance at 20 Hz, 20 KHz, and just for grins 100 KHz. There is also a screen that shows the input from my square generator on the bottom blue trace against the preamp output on the top trace at 100 KHz. When we get together I'll explain the significance of the measurements.

Another important parameter is how gracefully any circuit behaves if driven into clipping. There is an image of a 20 KHz sine wave driven into clipping. The level is over 14 volts peak which translates into roughly 10 volts RMS, about five times the level that will drive most amps into clipping so there is tons of headroom. The circuit comes out of clipping exceptionally cleanly with no ringing. I'll explain why that is very important.

I took it over to David Berning as he has a reverse RIAA circuit to test the accuracy of the RIAA adjustment. Even without component sorting, the accuracy from 20 HZ to above 1KHz was spot on and there was a slight drop at 20 KHz of about 1 dB. Being very slightly low helps prevent harshness especially if the cartridge does not track well. Selecting component values would allow it to be closer but I have never done that because the variance in recordings often exceeds that. The overload level was 140 mV which will suffice for any cartridge/recording level I know of.

Right now, Saturday the 20th looks good for me to work on this. If not it will have to be in September. Please let me know.

Tom

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20 KHz square wave.jpg (102.14 KiB) Viewed 21107 times

100 KHz square wave.jpg (102.93 KiB) Viewed 21107 times

100 KHz square wave output top vs input bottom.jpg (110.11 KiB) Viewed 21107 times

[tomp]

Here are the other images.

Tom

Attachments

20 Hz square wave.jpg (102.1 KiB) Viewed 21106 times

20 KHz sine wave at clipping.jpg (109.95 KiB) Viewed 21106 times

[JDCrae]

Tom,

Cannot thank you enough for all your hard work. It looks like I may not be back from Vacation yet on the 20th, so it seems September will be the month. My apologies, I thought we were back the 19th, but apparently we are returning on the 20th. After 8 hours of driving I will be less than helpful.

Very much looking forward to it, as well as the explanation of all aspects. September cannot get here soon enough!

Thanks

Jeff

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How does September 10th look?

Tom

[JDCrae]

September 10th works fine for me !

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See you then.

[tomp]

Speaking of a DIY guy with time on his hands.

Tom's offer is very generous and he has many DIY scratch-build projects under his belt. You could learn a lot in a very short time.

Especially soldering small devices. You can easily "heat-fry" an op-amp if not careful.

Walt:

An interesting story on Op Amps that happened yesterday . When testing the board at David's we started with the right channel and noticed a problem including a 2 volt DC offset. I though I detected some heat so I very quickly touched one of the Op Amps and promptly burned my finger so badly I had a blister. I immediately turned the power off and checked the board. In my haste to get the power supply connected I had reversed the plus and minus wires to that channel. The power supply is one I have laying around that is quick and dirty with 78 and 79 series regulators. The regulators sensed the excessive current and went into limiting preventing damage to the PS. The wiring to the left channel was OK so we tested that channel after removing the Op Amps in the right channel and fixing the polarity so we did not blow up the bypass tanatalums.

The left channel worked fine. After the tests, just for grins I put the cooked Op Amps that were now cooled down back in the right channel and both David and I were shocked to find that they both worked perfectly. We could not believe our eyes. It was amazing that an IC so hot that it could raise a blister on your finger with just a quick touch could still work after cooling down. Of course both amps went into the trash because I would not trust something that had been that hot. Although the reverse voltage was limited on the bypass tantalums I replaced those as well for dependability reasons. The bottom line is that I would have put down a large sum of money that the amps would have to be total trash but they were not. Will wonders never cease. The take away from all this is check your wiring several times before plugging things in. When you rush bad things happen. This from your bonehead friend Tom. You would think I would know better after so many years.