Question For Formula Users

[glennfin]

I just got a Formula pre. The first few days I was using the record outputs into my console but yesterday I decided to check out the main outs and much to my surprise, I noticed the record output level has WAY more output than the main outputs.

Thinking I had a problem, I started troubleshooting. My day job is working as a repair tech for an industrial repair house. I have over 25 years experience in electronics both as a technician and an engineer. I actually ran a musical repair business back in the 1980's so I have quite a bit of experience especially with tube circuits.

After spending some time poking around (without a schematic) I came up with nothing. From what I could tell, the main outputs and the input to the record circuit (speaker sim circuit) are fed from the same point!.. There doesn't seem to be any reason why the record outputs would be so much higher in output than the main outs.

Well today, after several hours of searching on the net, I finally found a schematic and yes, indeed, the Main outs and the input to the record circuit are in fact fed from the same point!.
This one's got me stumped!.... The only thing I can think of as to why the record outputs are higher in lever than the mains, is the output of the record goes through a transformer and maybe that's stepping up the output level?... I don't know... Now that I have the schematic, I'll poke around again....

Question........ Any other formula owners notice this? maybe it's normal for the main outs to be so much lower in level than the record outs?....... :?:

Thanks!

GlennFin

 

[ando]

No, it's definitely not normal for the record outs to be louder. Normally it's the opposite - the record outs should be fairly low output because it's meant to be a line-level signal suitable for input into a desk, PA or recorder. You'll be cooking your input channel at that rate.

Similarly, the normal outs usually have a potentially high output so that they can push the input stage of a power amp (if you like that sort of thing).

Unfortunately I can't be of more help than that because I know nothing about the Formula. Is there any evidence that your Formula has been modified?

 

[glennfin]

Well I'm not absolutely sure yet but I might have to file this one under "activate brain before placing mouth in gear" . After looking at the schematic and thinking about it again, I realized that the Record outs are buffered by a 2 stage op-amp circuit in addition to being transformer output coupled. The main outs on the other hand, come directly off the plate of the last tube stage through a .047 mfd capacitor.

In other words, the Main Outputs are not buffered and probably need to be connected to a fairly high impedance load to achive full output (like the input of a tube power amp). The Record outs, since they're buffered, can connect to a lower impedance load, like the line input of a console.

When I was checking the difference in outputs, I was checking both outputs into the input of my console...... That may be why I'm seeing a difference in levels.... Since the mains outs and the input to the record out circuit are connected to the same point, there isn't much that can be wrong in the preamp....

Now off to experiment again.... :roll:

 

[glennfin]

This is looking more and more like a question of impedance matching...

The line inputs to my console (Yamaha 02R96V2) are 3k ohm input impedance.

The Line inputs to the tube power amp in my guitar rack (Marshall 9100) are 30k ohm input impedance.

The line inputs to the Mesa 50 50 tube power amp appear to be 250k ohm input impedance.

The line inputs to my solid state power amp (QSC RMX850) are 10k ohms input impedance.

Rule of thumb, the output of a device should always be a lower impedance that the input it's connected to. Being that the Main outs from the formula are coming directly off the plate of the last tube stage and are not buffered, that's probably a relatively high output impedance and needs to be connected to a much higher impedance load than my 3k ohm console line input.

The reason I wanted to connect the main outs from the formula to my console is to record clean guitar tones without the high frequency cutoff that the record circuit provides. The solution to this will be to build a simple buffer circuit to connect the main outs to the console.

8)

 

[ando]

You could try just using the FX loop send for your purpose. Of course, that may mean you miss a tube stage (I don't know how the Formula is set up), but it would be like using REC out (Line level) but without the high frequencies being cut.

Are you using a virtual speaker cab on your computer?

 

[glennfin]

Nope that wouldn't work, already tried that and the send level is about the same as the main outs. It wouldn't be like the record out because the record out is a buffered signal.

As I mentioned, the reason the record out is louder when plugged into a line input on my console, is because it's buffered and the last tube stage doesn't get loaded down like the main outs which come directly off the last tube stage.

What I'd like to do is add a simple buffer circuit inside the Formula to buffer the main outs but also include a bypass switch to allow me to switch back to the stock configuration.


I'm not using a computer to record. I'm using a Yamaha 02R96V2 console and an Alesis HD24.

You could try just using the FX loop send for your purpose. Of course, that may mean you miss a tube stage (I don't know how the Formula is set up), but it would be like using REC out (Line level) but without the high frequencies being cut.

Are you using a virtual speaker cab on your computer?

 

[ando]

Nope that wouldn't work, already tried that and the send level is about the same as the main outs. It wouldn't be like the record out because the record out is a buffered signal.

As I mentioned, the reason the record out is louder when plugged into a line input on my console, is because it's buffered and the last tube stage doesn't get loaded down like the main outs which come directly off the last tube stage.

What I'd like to do is add a simple buffer circuit inside the Formula to buffer the main outs but also include a bypass switch to allow me to switch back to the stock configuration.


I'm not using a computer to record. I'm using a Yamaha 02R96V2 console and an Alesis HD24.

Really? The send level for an effects loop should be well below the main outs - well it is for most preamps anyway given that they usually have another gain stage post loop. They usually do it that way to avoid clipping or frying your effects units. It's certainly the case for my Triaxis and JMP1 preamps and it just makes sense. But you have a Formula in your possession so you'd know more about it than I would. I think your unit has some faults though. This just isn't adding up right.

I'm a little surprised that you aren't doing anything about speaker cab simulation. From my experience, just recording a plain uncoloured signal from a preamp sounds horrible, and applying EQ in post doesn't capture the speaker simulation aspect either. I think computer speaker cab plugins are great! They capture the non linear aspect of speaker cabs and offer many parameters which are intuitive to use. You obviously have devised your own methods though.

Anyway, I don't think I'm helping much so I'll shut up now. Best of luck though!

 

[glennfin]

"The reason I wanted to connect the main outs from the formula to my console is to record clean guitar tones without the high frequency cutoff that the record circuit provides. "

 

[ando]

You're actually not all that friendly, are you? Good luck, I'm signing off.

 

[glennfin]

Actually, I've been told I am quite friendly and chicks dig me.. :lol: :lol: :lol:

You're actually not all that friendly, are you? Good luck, I'm signing off.

Regarding speaker sims, that's what the recording output on all the Mesa preamps is, a speaker simulation. They call it a Record output, but it's meant to simulate at least part of the effect of playing through a guitar cabinet which is high frequency roll-off.

Yes, if you're trying to record distorted guitar tracks directly, you wouldn't use the main outputs because like you mentioned, the sound would be shrill, nasty, fizzy, etc...

If I want to record distorted guitar tracks directly into my console, then I would definately use the record output from either my Formula or Triaxis. If I want to record clean guitar tracks (no distortion) then I would want to use the main outputs because they don't attenuate the high frequencies like the record outs do.

Besides using the Mesa preamp record outputs to record direct, I have several other methods I use to record guitar tracks. I have a Digitech GSP1101 in my rack that has some very nice speaker sims, I also have a makeshift isolation room adjacent to the main studio room. I ran a snake through the wall into that room so I can run the speaker outputs from my tube amps in the main room and mike lines back to the console...... for recording real cabinets with a mike.

 

[glennfin]

Yesterday I built a simple buffer circuit using a 5532 op-amp. Connected it between the Formula main out and the console line input..... sure nuf, the main output was now the same level as the record out. That proves the Formula's main outputs were being loaded down by the console's 3K ohm line input.

Nothing wrong with the Formula. I decided to put the buffer circuit into a small box, external to the Formula..... Really didn't want to modify the Formula. :wink: