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PostPosted: 29 Dec 2020, 15:37 
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Hi, That is correct.

Good listening
Bruce

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PostPosted: 31 Dec 2020, 18:04 
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gofar99 wrote:
Hi, That is correct.

Good listening
Bruce


You are definitely correct. My wiring on my PCB isn't! I somehow installed the 100uf electrolytic on the first stage and not the second stage of the lower triode across the 680R resistor.

While I was testing out capacitors I noticed this mistake. It did sound better when I bypassed the 70K Resistor with a 0.47uf capacitor (lots of low end but still missing some high end) I am assuming that the high frequencies are just going to ground through the 100uf cap in the lower triode in the first stage? I am almost certain this is the mistake I am looking for as the circuit is built on a PCB I made so the only thing that is different in this upgrade I have done is change the resistors and add the 100uf cap for what I was looking for, a slight increase in output.

I shall have to remove the PCB and either move it over, or leave it out.


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PostPosted: 31 Dec 2020, 20:48 
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Hi, Say what? a .47uf across the 70K. Way too much. It will throw off the RIAA equalization big time. The resistor might as well not be there at all. Something in the range of 100 to 470 pico farads will restore some high frequencies. The 100ufs on that cathode have little effect on the frequencies at either end of the spectrum. What it will do is slightly alter the impedance of the stage's output. That will alter the equalization a bit as well, but not nearly as much. The change using the 100uf is about 5000 ohms lower than without it. The stage's internal impedance is figured into the value needed for the resistor. So when you have a capacitor on the cathode that large (very small ones don't really matter in the calculation) you should increase the value of the 70K resistor to about 75K. Without the capacitor the 70K is correct. The capacitor will also increase the stage gain by about 3 db. I personally prefer the sound without it. One less non-linear component to alter the flavor of the preamp. They all can do it and I want to minimize it as much as possible. I figure as long as I am not a full gain on my main preamp, I don't need to worry about what the phono preamp is doing. That is what volume controls are for.

Good listening
Bruce

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PostPosted: 01 Jan 2021, 00:01 
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Indeed, 0.47uf is not correct where I placed it while I was checking things out, while a record was playing to hear what the changes might be. With a 70k in series with 0.22uf and a 100uf cap across the cathode resistor in the first stage, I have almost no treble. Putting a range of 150pf - 500pf across the 70k didn't give much treble boost. I'll remove the 100uf capacitor in the first stage and listen again.


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PostPosted: 01 Jan 2021, 08:45 
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Hi, Based on what you indicate I suspect a wiring error between the first and second stages. A 470 pf cap placed across the 70K makes a noticeable difference. On a scope I get a 3db rise from about 5KHZ with the 470. This is really audible. The removal or addition of the 100uf on the cathode will only change the impedance and thus the RIAA equalization less. It will shift the trun over point for the low end by about 200HZ. It might not be audible.

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Bruce

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PostPosted: 01 Jan 2021, 18:57 
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gofar99 wrote:
Hi, Based on what you indicate I suspect a wiring error between the first and second stages. A 470 pf cap placed across the 70K makes a noticeable difference. On a scope I get a 3db rise from about 5KHZ with the 470. This is really audible. The removal or addition of the 100uf on the cathode will only change the impedance and thus the RIAA equalization less. It will shift the trun over point for the low end by about 200HZ. It might not be audible.

Good listening
Bruce

You were correct, Bruce.

I had somehow had several incorrect resistors in the entire circuit. Went through checking all of the resistors and replaced what wasn't correct and also left out the 100uf capacitors.

I can say that it all functions perfectly now and have ample treble without the need for bypassing the 70KOhm resistor. Thank you for your help!

I do have one question though regarding the woofers. At a normal listening volume, the woofers function just fine with more than enough bass, but when I really turn it up, the woofers are still producing ample bass but they tend to float in and out. Do I need to isolate my turntable better from the speakers themselves? Would the tone arm be picking up the bass from the speakers and amplifying that signal as well?


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PostPosted: 01 Jan 2021, 20:17 
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Hi, I'm glad you sorted it out OK. I began a habit a while back of measuring each resistor and capacitor before installing them. I have found several that were suficiently out of tolerance to mess up sensitive circuits. I use an inexpensive LCR meter. With it I can get matches in the 1% range. I recall it being only about $50. A real life saver. A no brand one on Amazon.

I'm not sure what you mean by floating in and out. Please explain.

Good listening
Bruce

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PostPosted: 01 Jan 2021, 21:00 
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I have built your Kt88 mono blocks, many many other projects as well. Never have I ever had so many wrong values for resistors in a whole circuit. Which leads me to think I wasn't even looking at your diagram.

It's all sorted now, that's all that matters!

About the woofers: with the volume turned up, the woofers oscillate in and out a lot. No odd noises, it just doesn't seem to be a good thing. At normal volume levels, the woofers operate normally.

My current setup on how I have my turntable, speakers and everything else audio, they all share a long narrow table with a speaker beside the record player.

I am pretty sure that the low frequency from the woofer is getting into the tonearm and creating the problem,which only presents itself at high volume.

I need to isolate the turntable from the speakers and possibly play around with turntable placement.


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PostPosted: 01 Jan 2021, 21:55 
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Hi, if it does this with no signal, just having the volume turned up then it is likely to be low frequency feedback like you suspect. A rather common problem when the turntable is not well isolated. The phono preamp does not have a low frequency filter as I feel they later the sound in subtle ways. There is significant gain below the audio band so it can occur.

Good listening
Bruce

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PostPosted: 01 Jan 2021, 22:47 
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Thank you for help, Bruce.


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