JCM 800 guitar preamp to mic preamp

[Amled87]

So, I've now built two operational JCM 800 preamps that I think turned out awesome with help from here!

My question is, could I put the 1M input resistor on a switch so I could change from high impedance to low impedance so I could use the preamp as an unbalanced mic preamp?

I plan to use XLR/TRS jack for the input if so.

Would putting the 1M input resistor on a switch with a 500-Ohm resistor work? With the switch I could swap between 1M and 500-Ohm, use the 1M for guitar and the 500-Ohm for dynamic mics? I've attached the preamp schematic I've used for both of my builds.

 

[Bo Deadly]

Yes.

Or you could make the switch parallel a 500 ohm resistor with the 1M. You never want the grid to be an open circuit, even for an instant, because you'll probably get a loud popping noise when engaging / disengaging the switch.

Note that the audio will be high passed, distorted and noisy.

 

[Amled87]

Yes.

Or you could make the switch parallel a 500 ohm resistor with the 1M. You never want the grid to be an open circuit, even for an instant, because you'll probably get a loud popping noise when engaging / disengaging the switch.

Note that the audio will be high passed, distorted and noisy.

Even if I kept the gain low and master volume high it would still be distorted? For the high pass, I could make the .68uf cathode bypass and treble peakers switchable as well. Would i be better off building something 2-3 stages with the cathode biased in the middle (say 1.5k)

Just brainstorming here, and thought maybe I try the JCM 800 preamp for a different purpose.

 

[TheJames]

Even if I kept the gain low and master volume high it would still be distorted? For the high pass, I could make the .68uf cathode bypass and treble peakers switchable as well. Would i be better off building something 2-3 stages with the cathode biased in the middle (say 1.5k)

Just brainstorming here, and thought maybe I try the JCM 800 preamp for a different purpose.

Understand that there's quite a bit of frequency shaping in a guitar preamp. For instance, the cathode bypass of the second stage is going to favor midrange and up frequencies. Prior to that, the coupling cap between the first gain stage and the second gain stage will further high pass. This continues on through the tone stack etc. If I recall correctly, the "flattest" position of the tone stack will be all knobs full up.

If you were to turn the gain down to the point where the preamp did not distort, you might have a really high level of noise that might be undesirable.

All that said...As long as you're not expecting it to sound like a nice tube mic pre like a Manley or something, you could have something that's fun to use.

Nothing wrong with trying.

 

[Amled87]

Understand that there's quite a bit of frequency shaping in a guitar preamp. For instance, the cathode bypass of the second stage is going to favor midrange and up frequencies. Prior to that, the coupling cap between the first gain stage and the second gain stage will further high pass. This continues on through the tone stack etc. If I recall correctly, the "flattest" position of the tone stack will be all knobs full up.

If you were to turn the gain down to the point where the preamp did not distort, you might have a really high level of noise that might be undesirable.

All that said...As long as you're not expecting it to sound like a nice tube mic pre like a Manley or something, you could have something that's fun to use.

Nothing wrong with trying.

I've picked up some vishay foil resistors, low noise wirewound and etc to try. Alot of the noise from these preamps come from the 68k resistors on the grids. As I said, I've picked up Vishay foil and wirewounds to hopefully aid in eliminating noise. I may also try a few different value coupling caps, treble peakers values, or removing them all together as well as changing or removing the "bright cap" on the volume pot.

My biggest question was rather I could use a switch (1k top, 500-Ohm middle and 1M bottom) for the input so I could switch between high impedance for guitar and low impedance for a dynamic microphone.

I'm definitely not expecting this to be equivalent to something like a Manley, Telefunken or etc preamp. Just thought it could be potentially cool for something to add some "Color" to vocal track using a dynamic mic.

 

[Tubetec]

You could try a mic to grid step up transformer at the input to better match dynamic mics .
The combi XLR/jack could be wired so on XLR its via the transformer on jack its hi-z direct to grid.
DC heaters would most likely improve matters with regards noise if you havent done it already .

 

[ccaudle]

I've picked up some vishay foil resistors, low noise wirewound and etc to try. Alot of the noise from these preamps come from the 68k resistors on the grids.

Resistors generate two types of noise. Johnson noise or thermal noise is intrinsic to any conductor, and is proportional to resistance and temperature.
All 68k resistors will have the same amount of thermal noise at a particular temperature.
There is an additional noise component which is affected by how the resistor is constructed and the current flow in the resistor.
Since the grid resistors have very little current flowing through them, I would expect minimal improvement in noise from changing out the 68k resistors.
The plate resistors seem like a good candidate for swapping with low noise variants, but I haven't worked with tubes, it is possible that the noise current from the tubes themselves swamps the contribution of the resistors. You could just try it and see, but I am sure there are some analytical texts around which have analyzed the various noise sources in tube circuits.

My biggest question was rather I could use a switch (1k top, 500-Ohm middle and 1M bottom) for the input

As previously mentioned it would be better to leave the 1M permanently installed, and have the switch change between disconnected and lower resistance.
500 Ohms seems really low to me, most mic amps are around 1500 Ohms. Are you sure you want it as low as 500?

 

[Amled87]

Resistors generate two types of noise. Johnson noise or thermal noise is intrinsic to any conductor, and is proportional to resistance and temperature.
All 68k resistors will have the same amount of thermal noise at a particular temperature.
There is an additional noise component which is affected by how the resistor is constructed and the current flow in the resistor.
Since the grid resistors have very little current flowing through them, I would expect minimal improvement in noise from changing out the 68k resistors.
The plate resistors seem like a good candidate for swapping with low noise variants, but I haven't worked with tubes, it is possible that the noise current from the tubes themselves swamps the contribution of the resistors. You could just try it and see, but I am sure there are some analytical texts around which have analyzed the various noise sources in tube circuits.



As previously mentioned it would be better to leave the 1M permanently installed, and have the switch change between disconnected and lower resistance.
500 Ohms seems really low to me, most mic amps are around 1500 Ohms. Are you sure you want it as low as 500?

500 was just a number I threw out there, however the impedance would need to match the dynamic mics I will be using so 1.5k would be ok. I've since started looking at different ways of doing things and getting ideas.

All of my 2204 components are on a turret board so easy swapping. I've since put the 0.68uf cathode cap on a switch so I could bypass it to get a "fuller sound" as the extra high mids/highs aren't exactly useful as well a 2.2uf cap. I've looked at swapping some of the .022uF coupling caps out for higher values to tailor the frequency response as well.

I've also made the input switchable between 1k5, 10k and 1M. I placed all of the input resistors inside of a mu-metal can and used shielded wiring going to and from that as well. I can get a rather clean sound from the mic, or crank the gain and get some neat distorted mic sounds.