No need to make it balanced or with phantom power, I just want to add a canon input to this amp:
It's for Harmonica of course and yes it has an isolation transformer , how would you change the input circuitry (R6,R7 and C3) to make it less guitar and more harmonica 😉
An externally hosted image should be here but it was not working when we last tested it.
It's for Harmonica of course and yes it has an isolation transformer , how would you change the input circuitry (R6,R7 and C3) to make it less guitar and more harmonica 😉
Hi,
Depends a bit on what mic will you be using .
.... I'd rather change the wiring of the mic-cable than hacking the chassis.
Also consider an external adapter -- depends on what impedance your mics are.
For the "usual"(Hi impedance!) mics I'd double :
C3, C2,R1,R2 and eventually R7 (Astatic mics and similar)
I'd also try to "up" R 10 and R9 -- the lower the voltage the easier the
Input valve gets into Saturation.
(But hey -- do we really need this with a small amp like this -- with a pentode input valve ??)
Hope this helps
SigFire
....and dont't forget :
Mind security, you're working with potentially lethal voltages.
Depends a bit on what mic will you be using .
.... I'd rather change the wiring of the mic-cable than hacking the chassis.
Also consider an external adapter -- depends on what impedance your mics are.
For the "usual"(Hi impedance!) mics I'd double :
C3, C2,R1,R2 and eventually R7 (Astatic mics and similar)
I'd also try to "up" R 10 and R9 -- the lower the voltage the easier the
Input valve gets into Saturation.
(But hey -- do we really need this with a small amp like this -- with a pentode input valve ??)
Hope this helps
SigFire
....and dont't forget :
Mind security, you're working with potentially lethal voltages.
I doubt there will be enough gain with a microphone, especially low impedance.
Pin 2 of the XLR is hot or signal. Pin 3&1 connect together for ground.
This amplifier is dangerous and potentially lethal, heaters are connected to the primary mains supply!
Pin 2 of the XLR is hot or signal. Pin 3&1 connect together for ground.
This amplifier is dangerous and potentially lethal, heaters are connected to the primary mains supply!
Are you suggesting adding another gain stage?
There's an isolation transformer for the heaters.
There is no isolation transformer shown on the schematic and yes, you will need a 600R - 10k step up transformer from an old Unidyne mic or one is available from Shure Brothers, out of the high Z SM58.
See 10k to 600 ohm audio transformer microphone step-up step-down impedance hi-z | eBay for ideas.
That will give enough voltage for the amp to work with.
See 10k to 600 ohm audio transformer microphone step-up step-down impedance hi-z | eBay for ideas.
That will give enough voltage for the amp to work with.
The rest of the circuit is isolated by the transformer, but NOT the heaters. They are directly wired to the mains.
And before you modify the amp, why not get a mic impedance adaptor at any music store and try it.
Hosa XLR Female LO-Z to 1/4in TS Male HI-Z Microphone Input Impedance Transformer | Musician's Friend impedance adaptor&index=1
Hosa XLR Female LO-Z to 1/4in TS Male HI-Z Microphone Input Impedance Transformer | Musician's Friend impedance adaptor&index=1
^^^^ this is the shortest path.
Your specifying an XLR connector means you have a modern low impedance balanced microphone , so the above adapter or wiring a microphone input transformer is the way to go.
It will provide around 10X "passive gain" because of the turns ratio and need a mid/high impedance load, think making R6 100k or 220k .
Crazy using the same ID for different parts, but pull the 3 horizontal R6 and the jacks, wire transformer secondary straight to vertical R6, which will now be 100k/220k .
Since you are in Argentina, I suggest getting anyn old LEEA 87/88 microphone which had the transformer built-in and wires the XLR connector in a weird way: groun/unbalanjced LowZ/unbalanced HiZ ,and were meant to use a standard guitar plug at the other end of the cable.
Not the LEEA89 which was low impedance only and was wired ground/lo/hi like any other mic.
The second one is an 89 but is labelled "Impedancia dual" (dual impedance) meaning 250 ohms/20 k ohms , selectable at the connector.
OR: you can build a single Op Amp balanced input stage, to replace the transformer:
This one is double, because it uses a double Op Amp, you can build a single one with a TL071 .
R5/6/12/13 must all be the same, you can use 10K for 10x gain, 22k for 20x gain, both roughly what you can expect from a mic input transformer/adapter.
In that case, you can use any modern balanced mic.
As a side note: FORGET that DANGEROUS uninsukated heater wiring, which to boot will not work with Argentine 220V mains.
I understand what you want to do and suggest you build a Champ or equivalent for your harp.
Your specifying an XLR connector means you have a modern low impedance balanced microphone , so the above adapter or wiring a microphone input transformer is the way to go.
It will provide around 10X "passive gain" because of the turns ratio and need a mid/high impedance load, think making R6 100k or 220k .
Crazy using the same ID for different parts, but pull the 3 horizontal R6 and the jacks, wire transformer secondary straight to vertical R6, which will now be 100k/220k .
Since you are in Argentina, I suggest getting anyn old LEEA 87/88 microphone which had the transformer built-in and wires the XLR connector in a weird way: groun/unbalanjced LowZ/unbalanced HiZ ,and were meant to use a standard guitar plug at the other end of the cable.
Not the LEEA89 which was low impedance only and was wired ground/lo/hi like any other mic.
The second one is an 89 but is labelled "Impedancia dual" (dual impedance) meaning 250 ohms/20 k ohms , selectable at the connector.
OR: you can build a single Op Amp balanced input stage, to replace the transformer:
This one is double, because it uses a double Op Amp, you can build a single one with a TL071 .
R5/6/12/13 must all be the same, you can use 10K for 10x gain, 22k for 20x gain, both roughly what you can expect from a mic input transformer/adapter.
In that case, you can use any modern balanced mic.
As a side note: FORGET that DANGEROUS uninsukated heater wiring, which to boot will not work with Argentine 220V mains.
I understand what you want to do and suggest you build a Champ or equivalent for your harp.
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