effects loop idea

[daz]

yeah, i just found that out. I only had to swap the shielded cable to the treble out and return to the master. Just did that and no amount of DSP twiddling worked. Tone was thin and low. So on with the SS loop. I googled it for an hour this morning and was surprised there weren't a million schematics to add a SS loop, just those few mod boards for sale. So thats likely the way to go but first i'm going to think about building one. I've built a ton of stomp circuits so i'm no stranger to that. But i wouldn't have a clue how to design a circuit. I wonder if a clean boost stomp circuit would work with some input changes for impedance matching?

[Chuck H]

It's very likely the output impedance of a clean boost circuit is going to be low. As would the output of volume pedals and some other typical devices. The low impedance isn't the problem. Close should be fine. As in "low" is close enough. The trick is to have it designed for unity gain. That is, whatever goes in is amplified a fixed amount that tracks a duplicate of the amps signal source before padding. YOU will need to acquire that figure with measurements. I will explain how if/when it comes up.

[daz]

Well, i have a email into metroamps so while i wait for a reply if something should be posted i'll consider it. Otherwise i'll probably just buy the metro board. I know DIY would be dirt cheap and easy logistically but i gotta get this done so i'll likely bite the bullet for the metro.

[Chuck H]

And, honestly, WHY do you "gotta get this done"??? Impatience? You have other amps. Do you have a gig breathing down your neck that no other amp will work for? Nah. I'm going with impatience.

You could almost certainly pull the guts from something like this (below). Run it on rectified 5V supply and make it work with almost no modification.

Andoer XP Booster Electric Guitar Effect Pedal Mini Single Effect with Clean Boost True Bypass - SD Instruments

[daz]

But you said a pedal isn't going to have the correct impedance or level? I'd rather pay $70 and get what is from all accounts a perfect loop than risk $22+shipping for something that may be a compromise. If i were going that route i'd just build a clean boost and having most of the parts it'll likely cost me a couple bucks. But even then i don't know if it'll work.

[Chuck H]

But you said a pedal isn't going to have the correct impedance or level?

Nope. I said:

It's very likely the output impedance of a clean boost circuit is going to be low. ..."low" is close enough.

Which is pretty much the exact opposite.

That particular pedal has a 1k output impedance. About the same as your cathode follower, which is what you want. However, further research reveals that it probably won't have enough boost for the job You probably need about 15 to 20dB (adjustable for matching it to your particulars). I know there are clean boost circuits that would handle it and have a low impedance output.

The module you're looking at takes your signal, pads it down to a lower level at a lower impedance through a transistor circuit. Then it recovers the signal and boosts it back up with another transistor circuit. It has the advantage of a built in filter for the power supply, being purpose made and comes with installation instructions.

The idea I propose uses an already existing circuit in your amp for the padding and low impedance so there is only one transistor circuit for recovery. I only proposed it because I prefer simple, eloquent circuits and using your amp for the send with only a recovery circuit is more eloquent than cutting into the signal path where it requires an additional SS buffer stage. So, my proposal has the advantages of eloquence and cost. My proposal has the disadvantage of requiring YOU to build the circuit, including the filtered power supply, and certainly a little tweaking to get levels correct. But it's actually pretty simple stuff and you have access to help. You already have experience with making pedal circuits and this wouldn't be much different. All you need to do is locate a clean boost circuit good to +20dB with an adjustable output.

In the end the choice is yours. And buying the existing unit comes with no questions (well, a few). My idea comes with more questions, but they aren't without answers.

[daz]

Hmmmm...but if a clean boost pedal would work off the cathode if it had enough boost, why wouldn't my DSP work in that location just using a passive loop? The DSP has a 1k output impedance and a ton of level control with a input level knob and level meter on front, seperate 0-100% volume levels per patch and a global 200% output level. Should i just try that, and if so would i use 2 50k's in series for the cathode and use the middle to tap off for the send? Or a 1k between the 100k and ground?

[daz]

I just tacked it in like your schematic. Used 2 47k resistors for the cathode and set the input so the level was average. Had both patch and global outputs full up and very low volume. In any case, it DID accomplish one thing.....i was able to hear it and the tone was great. No degradation aside from the volume loss. Why is is a DSP can't even put out as much as a pedal, assuming that what you seemed to insinuate is true?v(that some clean boosts should, just not the one u mentioned)

[Chuck H]

200% is 3dB. You need 15 to 20dB and a much lower output voltage divider to do this "right". The DSP doesn't offer the necessary amplification. It's designed to deliver not much over 0dB of the signal put into it. When you set that level for the input of the DSP you are also setting the output. It wants to achieve unity gain +/- a small amount of user adjustment. This is why we need a recovery stage.

The reason for the higher fidelity is the lower impedance output direct from the cathode follower. The actual divider will probably be something on the order of 40 or 50:1 to achieve about a volt of signal. That's close enough to "line level" or what most processors call "0dB" (a misnomer as it applies to the input, but who cares) that you shouldn't have any problems with clipping yours or any other DSP unit. With this volt of signal the DSP will happily put OUT a volt of signal. So now we need the other amplifier circuit to bump it back up to your stock cathode follower output and impedance. That feeds your tone stack, which responds the same because the impedance is similar and Bob's your uncle. Easy peasy.

I'm still researching the easiest way to achieve this (with less intrusion to the signal chain and for less than 80 bones )

[daz]

Ahhh, i see now. I didn't realize the in and out were linked when you set the input level. Otherwise i think it would have worked. I guess i could likely put any clean boost pedal between the DSP and return and it should wok, as i believe most of them do provide about 20dB. I may be wrong but i seem to recall that being typical when i was building pedals years back.

[Chuck H]

Ahhh, i see now. I didn't realize the in and out were linked when you set the input level. Otherwise i think it would have worked. I guess i could likely put any clean boost pedal between the DSP and return and it should wok, as i believe most of them do provide about 20dB. I may be wrong but i seem to recall that being typical when i was building pedals years back.

I was going to ask if you had a boost pedal just to try that. Good idea. Of course, you can't run a loop that requires an external amplifier to always work. You do need to build something into the amp before it's over.

[daz]

Thats one direction i could go, just build a pedal that specs 20db at least and build it into the amp with a bypass switch. It would be especially good if i could find one that would work on 5v from that unused winding but they're all gonna be 9v. Maybe some lower voltage to 5 for tho for the IC and i could change that part of the PSU. Also, theres a second tap on that winding for 6.3v.

[daz]

This could be good as a return circuit assuming i could run it from the 6.2v unused tap or can i get 9v from B+ w/o a lot of hoopla? 20Db-audio-booster_Circuit Diagram World

[J M Fahey]

Forget any 9V powered thingie there.

The MAIN problem is not Impedance, but as Enzo has incessantly been saying from the beginning a signal VOLTAGE one.
250/300V fed tubes can provide some 90V RMS signals, any day of the week.

+/- 15V fed preamps can provide some 9V RMS at clipping, which still can drive a PI which needs some 2V RMS or thereabouts.

9V fed stuff can usually provide 1V RMS; 2V RMS best case, so if used in a passive loop between Master and PI will clip turning your expensive Tube amp into a cheesy SS squarewave distortion system, with a tube power amp reamplifying that signal and adding nothing of is own.
You might as well replace the tube power amp with an LM3886 or something.

Even worse if loop is between tone stack and MV, because it will SS clip all the time, (constant high voltage signal there), even if you turn MV down for home use.

Only proper (transparent) loop is one attenuating signal, say, 10 or 20X to feed your pedal or DSP unit and then reamplifying it same 10 or 20X, using a high voltage device fed 200 to 300V (tube or HV FET or even HV bipolar) so it can reach 80 to 90V RMS if needed ... just as if the loop or effects pedal weren´t there.

[daz]

Is Chuck's suggestion of putting it between the cathode follower making the 100k cathode a 100k total voltage divider and returning it to the tone stack a good location? I tried it just to see what the tone would be like even tho the output wouldn't get loud at all, and the tone sound perfectly fine. So would it be a good idea to use it like that running the DSP input from a divider at the CF and then try and find a 15v or more circuit as a return amp ? And if so, what would i look for as far as output specs? I don't know how voltage relates to dB levels, which is how they usually spec outputs from devices like that.

Also, if i must i'll just get this, but they are supposed to go between the TS and master. You;re saying thats bad, so should i stay away from this product? https://store.metropoulos.net/produc...ss-fx-loop-kit