Author Topic: Most Pt2399 delay designs boost signal?  (Read 180 times)

garfo

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Most Pt2399 delay designs boost signal?
« on: September 09, 2018, 03:15:41 AM »
Hi guys, I have been away from the forum and I'm sure this debate has been covered before.
Anyway, after twicking with a Delay that I have built before, a Sea Urchin, I have noticed that with some pedals the Delay acts as a boost but with others it remains at unity gain. I was explained that this was due to the amplification being made on the inverted side of the opamp. I have also noticed that the pedals that make the Delay pedal boost are the ones that have a pot (Master Volume) on the output stage.
My question is:
If I want to leave the design as it is, would adding a simple jfet buffer at the input of the circuit control this boosting problem?

madbean

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Re: Most Pt2399 delay designs boost signal?
« Reply #1 on: September 09, 2018, 04:24:57 AM »
Adding a buffer in front will not reduce signal output, in general. It comes down to the design of each delay circuit and how the clean and delay signals are mixed together. A master output might allow for boosted signal but since it's a volume pot you can set it for unity gain.

In the case of the Sea Urchin, getting unity gain out could be made simpler. Change the following:
R2 to 360k
R5 to 22k
And while you are at it add a 100k to ground between C4 and R6.
I own madbeanpedals (duh). I am part owner of Function F(X).

garfo

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Re: Most Pt2399 delay designs boost signal?
« Reply #2 on: September 09, 2018, 02:49:44 PM »
The weird thing is that I can get unity gain with a straight guitar plugged into it. On the mixing stage I have a trim instead of a fixed resistor to fine tune the output level. the problem is when I place a fuzz or any other pedal that has some level of gain of its own and has a pot on the output of its circuit the sea urchin boosts it's signal, a lot.
I have tried decreasing the gain on the first stage (instead of 360k I have used 180k on both) and compensating on the mixing stage but the compromise is to have the clean guitar signal lower than when the delay is on. So, right now I am sitting in between having the guitar boosting my signal if playing with a Fuzzface type of pedal, or having the Delay signal lower than my clean signal coming straight from my guitar.
Hence my question about placing a simple buffer on the input of the delay.
« Last Edit: September 09, 2018, 03:10:17 PM by garfo »

reddesert

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Re: Most Pt2399 delay designs boost signal?
« Reply #3 on: September 10, 2018, 12:23:46 AM »
When a circuit has different output volume with different inputs, often that means that the circuit has relatively low input impedance and is loading the input source.  This usually causes the volume to drop if the source has a high output impedance. 

In the case of the Sea Urchin, it appears to have an input impedance of 180K, the value of R2, the input resistor leading to the inverting op-amp.  (It's not strictly caused by the inverting op-amp itself, but that does lend itself to low impedances, see https://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-input-impedance.php.) This is a bit low compared to say a guitar amp, which would typically have an input impedance of 500K to 1 Megohm.  It could be loading your guitar, since the output impedance of a passive guitar is quite high (also depends on your guitar's volume pot and where you have it set).  So guitar into delay might have a volume drop.

It's possible that you set the volume trimmer based on the guitar into the delay, and then when you put another pedal in front you get the volume back, so it seems like a boost.  If this is the case (and I'm not sure from the information provided), then a high-impedance buffer at the input might help, but you have to be sure that it's a really high impedance.

garfo

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Re: Most Pt2399 delay designs boost signal?
« Reply #4 on: September 16, 2018, 04:00:32 AM »
quote author=reddesert link=topic=28506.msg276519#msg276519 date=1536560626]
When a circuit has different output volume with different inputs, often that means that the circuit has relatively low input impedance and is loading the input source.  This usually causes the volume to drop if the source has a high output impedance. 

In the case of the Sea Urchin, it appears to have an input impedance of 180K, the value of R2, the input resistor leading to the inverting op-amp.  (It's not strictly caused by the inverting op-amp itself, but that does lend itself to low impedances, see https://www.radio-electronics.com/info/circuits/opamp_basics/operational-amplifier-input-impedance.php.) This is a bit low compared to say a guitar amp, which would typically have an input impedance of 500K to 1 Megohm.  It could be loading your guitar, since the output impedance of a passive guitar is quite high (also depends on your guitar's volume pot and where you have it set).  So guitar into delay might have a volume drop.

It's possible that you set the volume trimmer based on the guitar into the delay, and then when you put another pedal in front you get the volume back, so it seems like a boost.  If this is the case (and I'm not sure from the information provided), then a high-impedance buffer at the input might help, but you have to be sure that it's a really high impedance.
[/quote]
To try to make it clear.
I plug a Standard strat straight into the Sea Urchin and I have unity gain when the pedal is engaged. Now, I have a Fulltone Soulbender Clone that I have been playing and also a Fuzzface Clone. With both of them I set the volume pot to match my clean signal in terms of volume. The problem is that, when I have any of the fuzzes engaged and turn on the Delay pedal there is a huge volume boost. Could this be a mismatch of the output impedance of both fuzzes (500k) and the Sea Urchin input impedance(180k)? Also, isn't R1 (1M) setting the input impedance?
« Last Edit: September 16, 2018, 04:03:40 AM by garfo »

reddesert

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Re: Most Pt2399 delay designs boost signal?
« Reply #5 on: September 16, 2018, 07:05:50 PM »

It's possible that you set the volume trimmer based on the guitar into the delay, and then when you put another pedal in front you get the volume back, so it seems like a boost.  If this is the case (and I'm not sure from the information provided), then a high-impedance buffer at the input might help, but you have to be sure that it's a really high impedance.
To try to make it clear.
I plug a Standard strat straight into the Sea Urchin and I have unity gain when the pedal is engaged. Now, I have a Fulltone Soulbender Clone that I have been playing and also a Fuzzface Clone. With both of them I set the volume pot to match my clean signal in terms of volume. The problem is that, when I have any of the fuzzes engaged and turn on the Delay pedal there is a huge volume boost. Could this be a mismatch of the output impedance of both fuzzes (500k) and the Sea Urchin input impedance(180k)? Also, isn't R1 (1M) setting the input impedance?

R1, the pull-down resistor, doesn't set the input impedance (I guess it's an upper limit to the input impedance). For an inverting op-amp, the + input is held fixed at the reference voltage, which means that the - input is also fixed at the reference voltage of +4.5 V, because op-amps try to equalize their inputs. That means your input is connected by a 180K resistor to the fixed +4.5 V. So the input impedance is 180K. The link I posted above to radio-electronics.com has a pretty good explanation of this and how it differs for a non-inverting op-amp.

The output impedance of a pedal like a Soul Bender or Fuzz Face that has a volume pot on its output is a little complicated, because it depends on where in the pot's rotation you have it set. The 180K input impedance winds up being in parallel with the lower leg of the volume pot, that is it's in parallel with pins 1 and 2 of the volume pot. (There is probably a coupling cap between the volume pot and the Sea Urchin input, but for AC audio the cap passes signal.) This can make the volume drop when you switch the second pedal on, which is not what you are reporting.

What's a little out of the ordinary is that you put in a trimmer to adjust the output level. I am wondering, of course I don't know, if the Sea Urchin's natural state with just a guitar plugged in is to drop the volume, you adjusted the trimmer to take that out, and now when you put the fuzz pedal between the guitar and delay, it's boosting.