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Old 30th July 2012, 09:12 PM   #1
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Default valves to replace opamps in cd player

I am trying to replace the opamps with valves.
I have a tda1541 based cd.
I have one of those china based 6n3 buffer kits .
it runs at 70dc.

It is unity gain .I want to change it so it amplifies instead.

does anyone know how to change cicuit design for this,
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Old 31st July 2012, 03:24 AM   #2
ChrisA is offline ChrisA  United States
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Originally Posted by intensate View Post
I am trying to replace the opamps with valves.
I have a tda1541 based cd.
I have one of those china based 6n3 buffer kits .
it runs at 70dc.

It is unity gain .I want to change it so it amplifies instead.

does anyone know how to change cicuit design for this,
You must have figured out that a unity gain tube "amp" has about as much "tube sound" as a length of wire. They need to amplify. So you are on the right track but you are missing a critical piec of information...

How much gain is required?

Your best bet would be to leave the op amp in place but reduce the gain in the op amp circuit to unity.

Then you would need a tube circuit with the same gain the op amp used to have. But what is that?

The next question is what will this new tube circuit be driving?
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Old 31st July 2012, 04:21 AM   #3
kevinkr is offline kevinkr  United States
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The current board is a cathode follower so slightly less than unity gain. You'll need to quite a lot of etch cutting to reconfigure as a common cathode gain stage - probably would be worthwhile to just build from scratch possibly on a piece of vector board.
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Old 31st July 2012, 04:38 AM   #4
ChrisA is offline ChrisA  United States
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Originally Posted by kevinkr View Post
The current board is a cathode follower so slightly less than unity gain. You'll need to quite a lot of etch cutting to reconfigure as a common cathode gain stage - probably would be worthwhile to just build from scratch possibly on a piece of vector board.
You are correct about that. PCBs are not good if you want to modify the circuit. Also 70V is not much and there are better tubes available for only a few bucks.

I built a basic line level booster with gain from scratch and put it inside a box that once held a CDROM drive. It uses a 12V AC plug-in power supply (AKA "wall wort") and inside the box is 12.6 volt transformer running "backwards" that steps the 12VAC up to 120VAC which gives about 145 volts DC which is enough to make the 12AX7 tube happy. The heater runs off the 12VAC. Cost is "not much". The little pre-amp tubes make so little heat that they can live inside the box, out of sight and protected from damage. The trick is finding an AC wall wort, most are DC and don't work for this. It is a very easy first tube project.

For this case I'd use two tubes 12AX7 for the common cathode gain stage. Use local feedback to control the gain to the required amount. This drives a 12AU7 cathode follower that in turn drives the output jack. Each tube has two triodes,use one for left and one for right.

Sell the Chinese buffer on eBay. Use the cash to build the above
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Old 31st July 2012, 07:09 AM   #5
AJT is offline AJT  Philippines
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6n3 is like a 5670 tube, a quick way is the replace r13, r16 with jumper, transfer the 12k load resistors to the plates, take the output from the plate instead and you are done....
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Old 31st July 2012, 01:43 PM   #6
TheGimp is offline TheGimp  United States
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One thing to remember if you remove the op-amp is that it normally is a low pass filter to help remove sampling artifacts.

If you remove/bypass the op-amp and replace it with a tube amp, you need to add LPF to achieve a clean design.

Otherwise you will be amplifying a bunch of hash as well as the desired signals.
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Old 31st July 2012, 05:25 PM   #7
ChrisA is offline ChrisA  United States
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Originally Posted by TheGimp View Post
One thing to remember if you remove the op-amp is that it normally is a low pass filter to help remove sampling artifacts.

If you remove/bypass the op-amp and replace it with a tube amp, you need to add LPF to achieve a clean design.

Otherwise you will be amplifying a bunch of hash as well as the desired signals.
The simplest way, maybe is to reduce the gain in the CD player by (say) a factor or 20 with a 20:1 resistive voltage diver. (a.k.a. "volume control") and then set your tube amp for a gain of 20. This way you leave the guts of the CD player alone.

There is a long tradition of this in guitar amps. They have multiple gain stages and each is cut down with a resister divider before going to the next stage. You can build up all kinds of distortion in this way. I'm a little serious - that "tube sound" you are looking for adds a percent or so of distortion, mostly second harmonic. You need gain to get that.
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Old 31st July 2012, 05:57 PM   #8
TheGimp is offline TheGimp  United States
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Actually, you can just bias the tube in a more non-linear region and get the results.

You can also load the tube more heavily (decrease the plate load) and you will get more distortion.
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Old 31st July 2012, 06:12 PM   #9
ide2003 is offline ide2003  Indonesia
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Hi intensate, I've been in your shoes before. If you want no op-amp all together
I highly recommends you to have ECDesign's common gate Jfet I/V and your current tube buffer. Very transparent sounding but some works on the PCB needed.

check here for my work on tubing out Sony CDP950 with TDA1541
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Old 31st July 2012, 06:33 PM   #10
TheGimp is offline TheGimp  United States
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6N3P with 12K anode resistor, 2K cathode resistor bypassed with 22uF, 150V supply:

Direct Newton iteration for .op point succeeded.
Fourier components of V(out)
DC component:-0.00162395

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 1.000e+03 6.654e+00 1.000e+00 -179.00 0.00
2 2.000e+03 3.767e-01 5.662e-02 91.29 270.29
3 3.000e+03 9.815e-03 1.475e-03 -179.08 -0.08
4 4.000e+03 1.824e-04 2.742e-05 93.33 272.33
5 5.000e+03 7.049e-05 1.059e-05 -177.93 1.07
6 6.000e+03 5.919e-06 8.895e-07 -89.71 89.29
7 7.000e+03 8.660e-08 1.301e-08 169.23 348.23
8 8.000e+03 4.890e-07 7.348e-08 -105.87 73.13
9 9.000e+03 2.011e-07 3.022e-08 -179.59 -0.59

Total Harmonic Distortion: 5.663559%
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