Solo Dallas - SVDS Replica  [traced]

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Re: Solo Dallas - SVDS Replica

Postby moonshiner » 11 Jan 2020, 21:01

caspercody wrote:Just a reference point for voltage contacts. so if I needed to connect to a specific voltage, I know where they are located on the board. As for building the vero, it means nothing.

Thanks . I have made several hundred vero pedals . That was messing with me a tad ha ha . My parts come Tuesday . I’m excited . Ha ha
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Re: Solo Dallas - SVDS Replica

Postby madras » 12 Jan 2020, 02:45

Hi there!

As I mentioned before in this topic I have already build mine one based on original schematic:
viewtopic.php?p=277360#p277360

I found out major part of the schematic around LM380 seems to be useless. I am talking about the part marked in red circle:
Image

1. First of all R10 and C15 creates low-pass filter with roll-off above 72.3 kHz (C15=1nF) or 15.4 kHz (C15=4.7nF). In case of guitar signal it does nothing except atteuating signal in linear way and adding noise.
2. Next we have R11 and R12 making pure voltage divider which attenuates signal and adds more noise.
3. Next we have LM380 power amplifier which was originally purposed for driving XLR transformer. It amplifies already attenuted singnal by 34 dB and adds a lots of noise!! Attenuation and then amplification of a signal does not make any sense.
4. R13 and C20 is just a Zobel network originally introduced to neutralize the effects of the XLR transformer inductance load. In our case there's no transformer anymore! So no need for Zobel circuitry, becasue it does not changes anything which would be audible. For reference see - https://trueaudio.com/st_zobel.htm

Based on my analysis, I removed all the mentioned part. Thanks to removal of mentioned low-pass filter and voltage divder I gained quite a lot of signal signal level and now no need for additional amplification with LM380. So the amp was also removed together with Zobel circuitry. NE570 compander output has built-in opamp / buffer which is strong enough to push Marshall Plexi pre-amp really hard!

Thanks to the modification I reduced noise a lot! SVDS sound characteristics remains the same as before. The only difference is now I have significantly less noise and a little less gain, but it is not a disadvantage for me, because the highest levels are unusable in original Schaffer Replica, becasue in my testing a signal started to clip with default 15 VDC power supply.

I hope some people find it helpful. If someone thinks the thing is different, then please explain why the part of circuit I am talking about is important. I am looking for interesting discussion :popcorn:

Best regards!

For this message the author madras has received thanks: 2
Ben N (12 Jan 2020, 10:02), sixthfloor (12 Jan 2020, 14:38)
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Re: Solo Dallas - SVDS Replica

Postby EddieTavares » 13 Jan 2020, 17:58

madras, Great job!!!

There is some components in that circuit that does not affect the tone like l1, LM117, D1, D2, C34, R14, R15, C27, C31, C32, the LM380 does a little overdrive but is almost irrelevant.
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Re: Solo Dallas - SVDS Replica

Postby ppluis0 » 13 Jan 2020, 18:36

Hi madras,

OK to discard the LM380 and associated circuitry. There was implemented to drive an transformer to have a true floating XLR.

I'm in doubt about the need to process the signal using a compander, since this operation was due to establish an RF link between the instrument and the amplifiers.

Please try to extract the signal right at C5 -avoiding the compander chip- and tell us if you hear any difference.

Cheers,
Jose
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Re: Solo Dallas - SVDS Replica

Postby madras » 14 Jan 2020, 01:47

ppluis0 wrote:I'm in doubt about the need to process the signal using a compander, since this operation was due to establish an RF link between the instrument and the amplifiers.

Please try to extract the signal right at C5 -avoiding the compander chip- and tell us if you hear any difference.

Cheers,
Jose


Jose, you may be right. I have also wondered how compressing 2:1 and then expanding 1:2 may affect the sound. In theory it should do nothing. In practice companders wasn't perfect those days, so it is possible it adds some harmonics. What I obsered NE570 starts to clip itself when input potentiometer set to around 60-70% on 15VDC power rail. I have also obeserved NE570 boosts signal noticeably, so getting signal straight from C5 will effect with less gain :D

What shapes the tone for sure IMO is preemphasis circuirtry at the beginning and linear boost in NE570 (and LM380 ofc) pushing preamp tubes to distort.

EddieTavares wrote:madras, Great job!!!

There is some components in that circuit that does not affect the tone like l1, LM117, D1, D2, C34, R14, R15, C27, C31, C32, the LM380 does a little overdrive but is almost irrelevant.

It is power supply so this is obvious. Let's focus on signal path.

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Re: Solo Dallas - SVDS Replica

Postby caspercody » 14 Jan 2020, 05:17

I put a SPDT switch in and connected one side to normal output and the other side connected after C5 and I did not notice much if any tone change.
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Re: Solo Dallas - SVDS Replica

Postby ppluis0 » 14 Jan 2020, 21:09

caspercody wrote:I put a SPDT switch in and connected one side to normal output and the other side connected after C5 and I did not notice much if any tone change.


I always believed that the use of a compander was not justified and it was just a sales pitch. :roll:

In other words, this results in a bad reproduction of an old device, as been proved with the elimination of the LM380

If any other member of the forum that has build a replica of this "replica" can do the same test with a spdt switch I would like to hear his results in this thread.

After all, one of the reasons for this forum to exist is to break down myths, isn't it ? 8)

Cheers,
Jose
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Re: Solo Dallas - SVDS Replica

Postby caspercody » 14 Jan 2020, 21:43

Here is a vero layout with out the compander and beyond.
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