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ArcticBreaze 14th October 2012 07:28 AM

effects loop, harp amp
 
1 Attachment(s)
here is a schematic of my harmonica amp I think its accurate, I want to put in an effects loop for foot pedals etc. but they cant go before the first stage of the preamp as this causes feed back on the mic. would i have to put in another valve stage to do this?, prefer not to put solid state parts in if i can help it. Can i put it in just before the pi?
someone please point me in the right direction.

Printer2 14th October 2012 02:23 PM

Your running one channel into the other and you have feedback with some foot pedals? Not a surprise. I don't think it would matter where you put the effects from a feedback perspective, can't just put them anywhere in an amp without thinking of what size of voltage you are hitting them with though.

In front of your amp you have a certain acoustical output from your harp and an output from your amp hitting your mic. You only get feedback when the output of your amp is louder in the mic than from the harp. You only have so much gain you could add to a system before you feed back. Whether you arrange it as 2X plus 10X plus 15X or 10X plus 2X plus 15X you still get the same gain. I don't think putting them in an effects loop will change much.

Simon B 14th October 2012 11:36 PM

Quote:

Originally Posted by Printer2 (Post 3201178)
..........I don't think putting them in an effects loop will change much.

Me neither, there are other more promising areas to look at for this though.

You don't mention what mic you're using, but that might be worth thinking about, as might a bit of experimentation with how you hold it.

If the feedback's consistently on one particular frequency, it might be possible to use a notch filter or parametric eq to kill it without changing your tone too much.

sreten 15th October 2012 12:03 AM

Hi,

The position of your effects won't change the overall loop gain that causes
feedback. In guitar amps it allows you to use the overdrive characteristics
of the input stage prior to sending the sound to the effects loop. Which
still may be useful for harmonica if that is part of your sound, and like
guitar you want that sound before the lush effects rather than after.

I'd suggest asking in a guitar amp forum how to add a loop, done properly
it can't do the amplifier any harm, and will make it more flexible. Might
not solve your overall feedback problem though without further tweaking.

rgds, sreten.

Compression and distortion build up loop gain, which every guitarist knows,
but expansion, which suppresses low inputs from the harmonica mike, can
help getting a decent dynamic range without feedback, noting that it means
subtle infections /asides have to played more aggressively to come out right.

Printer2 15th October 2012 12:12 AM

Actually I thought this place a guitar amp forum, might as well answer then. You could put a jack after the first stage as your effects should have a high impedance input, but again if you have a gain of say 40X in the input and you are feeding one volt into the amp, you could get 40V out which your effects might not like. The return would be the other input across the 1M resistor, just remove the 220k .

gingertube 15th October 2012 12:40 AM

Just before that first ECC803 stage is the best option but it will require another tube (or some SS). You need an effects driver stage to drive the effects out and to provide an impedance buffer from the volume control.
Then you need an effects recovery stage. With those 2 stages you can then decide if you want serial/parallel switch, wet/dry blend etc.

I would RECOMMEND you pay out a few dollars for Melin Blencowe's Book "Designing Preamps for Guitar and Bass". It has a good section on Effects Loops and has the schematics of what you will need.
I see that you are across the ditch from me but I bought my copy thru' Mal and Carol at Evatco in Queensland.

Another option is just to buy a kit such as this one from Kevin O'Connor
Effects Loops - - BFX - Best All-Tube Effects Loop Kit

I'm sure that there are other kits from other suppliers as well.

Cheers,
Ian

ArcticBreaze 15th October 2012 01:18 AM

Quote:

Originally Posted by Printer2 (Post 3201178)
Your running one channel into the other

hey I don't know why this works, but it does. with the first triode turned turned way down about 2,3 and the second all the way up I can stand right in front of the amp putin out about 35 watts and no feed back, can almost put the mike upto the speaker before it starts to screech, but turn the primary Chanel up and the second down feedback city.(when i find the sweet spot will replace the first pot with a fixed resistor.)
with the upgrade from 6v6 to 6l6 I doubled the output still no feedback in fact it's better than it was. so this is why i think an effects loop may help. get the pedals after the primary triode.

Mic is a mad dog harp mic from "shaker" Shaker Microphones and Promotions, Inc.

Printer2 15th October 2012 01:19 AM

Why would he need a recovery stage, he has a full blown 30mV to full output preamp stage on the input stage he is using for the second stage. Depending on the effects he is using the first stage and how much cable he has in between them it might be just fine to drive it. Might get a little roll off of the highs if it is a long run but then again this is harp.

I would just give the simple loop a try if I really wanted to see if putting the effect after the first stage will effect the feedback. If it does not change things then putting a full blown effects loop in place is a waste of time as compared to running them up front. That is unless you wanted to send a distorted signal into the effects.

Printer2 15th October 2012 01:22 AM

Quote:

Originally Posted by ArcticBreaze (Post 3201893)
hey I don't know why this works, but it does. with the first triode turned turned way down about 2,3 and the second all the way up I can stand right in front of the amp putin out about 35 watts and no feed back, can almost put the mike upto the speaker before it starts to screech, but turn the primary Chanel up and the second down feedback city.(when i find the sweet spot will replace the first pot with a fixed resistor.)
with the upgrade from 6v6 to 6l6 I doubled the output still no feedback in fact it's better than it was. so this is why i think an effects loop may help. get the pedals after the primary triode.

Mic is a mad dog harp mic from "shaker" Shaker Microphones and Promotions, Inc.

Sounds more like you turned a low gain amp design into a high gain amp which is just on the edge of stability.

ArcticBreaze 15th October 2012 05:17 AM

Quote:

Originally Posted by Printer2 (Post 3201897)
Sounds more like you turned a low gain amp design into a high gain amp which is just on the edge of stability.

low gain amp WAS on the edge, high gain is maybe three steps back from the edge @ the same output, but turn her up sum more and you can play the feedback just like a guitar. (jimi hendrix harmonica lol):D

thinkin maybe a spring reverb tank maybe all the extra effects i need. can get the amp as dirty as I want it (Which is cleaner than most), but i do like a bit of reverb.

foot controllable reverb would be real nice. (pedal with a pot in it? like a sowing machine.) thoughts?


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