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Old 22nd February 2017, 07:04 PM   #1
ALPUY is offline ALPUY  Uruguay
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Default Attempted reverse engineering.

Hi. I tried unsuccessfully to copy the circuit of a known phono preamplifier.
I've captured it in LTSpice, but simulating it with reverse RIAA produces a graph that has nothing to do.
To simulate I have used ICs that are not real ones.
It's Pr. Je .. Phono B.x AC, dual mono that I got in garage sale at U $ S 5.
If someone suggests where the copy is wrong I thank you.

Click the image to open in full size.

regards
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Old 23rd February 2017, 03:00 AM   #2
PRR is offline PRR  United States
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> produces a graph that has nothing to do

What DOES it do?

But the "obvious" question is: why no other connection to U1 "- In" pin? We would expect the output of U1 to connect back to its "- In"; basic op-amp connection.

Suggestions: Lose U3 et al. This is a DC servo to correct real-world voltage errors. SPICE opamps have zero error. You do not need U3 to get it to "work" in SPICE. Lose the whole U2 stage. This IS needed to get the 500Hz+2KHz wiggle in the RIAA curve, but U1 alone will get a slightly-wrong but nearly-right response IF it is connected correctly.
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Old 23rd February 2017, 02:33 PM   #3
ALPUY is offline ALPUY  Uruguay
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Hi, PRR. I've listened to you.
I have removed the servo dc and have corrected errors in the copy.
I think now the frequency response agrees.
It is a Pro Ject Phono Box MM / MC preamplifier with AC power 16 volts.

Click the image to open in full size.

Thankyou
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Old 23rd February 2017, 02:39 PM   #4
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Looks very unclear for me.
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Old 23rd February 2017, 05:03 PM   #5
ALPUY is offline ALPUY  Uruguay
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If I have correctly copied this circuit (components that convert it to MC are not included), can there actually be this response simulating with AT 440MLb cartridge, with that peak at 20khz ?. You can have that accentuation in low frequencies so important.
The peak at 20khz disappears if you remove the 22p capacitor that is in series with the 47k load resistor.
Click the image to open in full size.

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Click the image to open in full size.
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Old 23rd February 2017, 05:12 PM   #6
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Saludos "Yorugua" :-)

Sorry, I don't understand the point.
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Old 23rd February 2017, 05:22 PM   #7
kevinkr is offline kevinkr  United States
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I suspect the schematic in this article may be somewhat similar to your phonobox if not necessarily identical. It is at least a starting point. (Yours appears to be newer)

I'm not sure what to think of this design..

Here: http://www.angelfire.com/az3/dimitri...s/phonobox.pdf
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Old 23rd February 2017, 06:09 PM   #8
ALPUY is offline ALPUY  Uruguay
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Kevinkr, almost identical. I was missing a resistor on U2. I put 47k parallel to 68n and the result is this. Thanks for the input.
For Osvaldo, greetings neighbor.

Click the image to open in full size.

Anyway, it's just curiosity. In sound performance I will stay with VSPS 300 and with Muffsy Kit.
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Old 17th March 2017, 05:49 PM   #9
ALPUY is offline ALPUY  Uruguay
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Hello again. I have successfully copied the circuit of Pro Ject Phonobox AC.
It is very similar to the circuit provided by Kevinkr.
I have made some modifications suggested by a circito that I found on the internet.
I show both circuits.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Of course I'm going to change the most beautiful aluminum pouna steel case, and I'll make a PSU with toroidal transformers + 15v 0 -15v / 50watts.
Sounds very nice.

Last edited by ALPUY; 17th March 2017 at 06:07 PM.
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Old 20th March 2017, 10:03 PM   #10
ALPUY is offline ALPUY  Uruguay
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Hello, searching the internet I found a circuit identical to that of Pro Ject Phonobox. It's apparently from a kit called Actidamp MK2.
I have already modified the Pro Ject based on the small variations shown in this circuit. Sounds great and reading the specs I found today on this page, it looks like it's good.

http://www.elektrolukis.estranky.cz/...amp-mk.ii.html

What will be the original circuit, Pro Ject or Actidamp MK2

Last edited by ALPUY; 20th March 2017 at 10:07 PM.
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