Calrec1061 PREAMP redesign schematic - thoughts?

Hi guys,

I'd really appreciate it if any guru's can give some feedback on the revisions to the original design.

Essentially I have added:
Local regulation
Slow-start 48V (I hope)
Full output fader plus a third gain stage.

The EQ (once sorted) should patch in before the fader for a complete channel strip with hardwire EQ bypass. The preamp should have 75dB of gain and I think I've managed to find a way for it to work with a single pole 12 position switch, so Lorlins can be used throughout keeping the cost way down. A toggle is used to select gain ranges via the 2nd stages static gain...

Here is the original schematic.
Here is my new one (PDF). [Edited to correct the regulation error]

So far I am unsure of a couple of things - the phantom bypassing (C15, C16 and R29). How much will be needed if the supply is bypassed at the regulator?

Will my slowramp 48V+indicator LED work as I imagine - lighting slowly as the 48V reg ramps? (R36 and D10 etc).

The T-pad set to 20dB attenuator between stages 1 and 2 - should be fine right? I tried to get close to the loading of the original.

I'm also not certain about the value of C9, should it be higher like 470uF instead of 10uF as the value of R12 is quite low (2k32)?

Any thoughts would be fantastic.
Cheers Tom

This is a rather complex design and I need more time to study it in detail but for the moment:
* the value of R3 looks wrong
* why D6 in series and not in parallel with C19?
* C15/R29 look OK
* R13 looks unnecessary

Can you proviede a schematic for the opamps? Many aspects of the design depend on them.

Samuel

Hey Samuel,

R3 sits in parallel with the switch values and sets the gain of the first stage, the feedback resistor is located in the amp I think and its 5k1 (R2 in this schematic).
Here is a HTM spreadsheet to show the gain calcs...

Hmmm D6 and D13 - I think I missed a connection on the schem to place them in parallel with R31/38 etc but I don't think the diode goes in parallel with C19/27? [Edit: Corrected the schem now...]
R9 & 13 are in the original schematic, basically any point where the device connects to the outside world. Are they not needed for the DC filter?

Thanks a lot
Tom

in the schematic it says R35 and C17 adjust ramp speed.. shouldn't that be R35 and C23?

I'll look harder as time allows. :thumb:

[quote author="Svart"]in the schematic it says R35 and C17 adjust ramp speed.. shouldn't that be R35 and C23?

I'll look harder as time allows. :thumb:[/quote]

Yup - bugger thats what happens when I go renumbering all the parts as I change the schem...

-T

The feedback resistor is located in the amp.

Click to expand...

OK, I see.

R9 & 13 are in the original schematic.

Click to expand...

R9 is fine--it keeps C7 at a well defined DC level if nothing else is connected to it. R13 is IMO unnecessary, as it is paralleled by the transformer which has much lower impedance. It doesn't hurt, just one component to save.

Samuel

Cool I'll remove it - thanks Samuel.

Tom

Hey Tom, the design looks nice and with 4 transistor amp-block!! I will
try it out when time comes. My thought about the gain-structure; I think you have enough gain. Your problem could be the Line-input; if you put +18 dBu (normal 0dB fs)
at your input, then you have about + 24 at the secondary and if I read it right +34 after the 202 amp. Can it deliver that ??. Another thought is
that my experience say´s that a toggle jump of 20 dB is rather much.
If you have cans on and move +20dB when the musicians( or your self)
are playing they/you are gonna jump. I know that 20 dB as a pad are normal but in a live-situation you normally start from left to right. If the
gain isn´t lo enough you pad and then you move on to get your gain settled. I would have tried with +/0/- 10 db instead. Perhaps you can go
up with a 5 dB gain in the last block. But then your insert is 5 dB lower than the output which not is unnormal.
The "nevish"way, helps you a lot. It turns in the second gain stage
when you need it, in one knob. I´ts more foolproof but it costs you a double 2x 12 switch.
Cheers Bo

I correct my self. The Tx gain is 0 and amp gain is 6 dB when Line input is used. That leaves you with +24 at the output of the first ampstage, or??
Can the input of the first stage handle +18 dBu??
Cheers Bo

Hey Bo,

RE the gain structure, you are right to notice this - the TX gain is 0dB and the first amp (202) provides 5dB at the lowest setting, so a +18dBu input would be +23 after the first amp....You are right it is hot. I tried to get close to matching the original style of operating level.

The combination of no TX gain and post 1st stage pad set the operating level for line input on the original. So this re-hack only adds 5dB exta gain in the 1st stage.....I'm not sure if the amp can deliver +23dBU but it would have had to deliver +18 in the original anyway...

I also agree that a toggle jump of 20dB is a bit big but I was curious to find a way that could use a cheap 1pole12pos switch like Lorlin. In theory its quite nice but operation would most likely be a different matter for sure.

In fact my first revision has a 20dB pad prior to the TX on line input only so the amp could handle massive +20dBu line signals....this was in combination with a 2pole12pos switch doing 5dB steps via both stages ala neve - it is more elegant.

I know US guys can get a 2pole12pos cheap switch from Mouser but I can't find anything suitable in the UK without going for Elma, Grayhill etc, so was trying out ways to save money. A £1 Lorlin cuts costs a lot compared to a £20 Grayhill when you're hoping to build 8 of 'em!

My original plan was to get a custom order of Grayhills from EAO...maybe a group buy for these and input TX's when the project's done?

I'm too busy mixing this week but I'll try and get an updated schem up with the original switching asap.

Thanks a lot for your thoughts, they were exactly what I was looking for!:thumb:
Cheers Tom

I studied your schemo a little further and didn't found anything that looks wrong, but here a few suggestions:
* I'd choose VR1 = 10k and R11 = 100k, this would lower noise a tad and easen the search for the pot.
* are TRS-inserts grounded to audio ground a clever idea? I'd at least add a 1 nF to chassis for the return for rudimentary RFI protection, idealy with a 1k in series ahead.
* what about a transformerless second output? Take another 33 ohm from C10 to hot and a 33 ohm to ground for an impedance balanced out, which gives a slightly different colour option and doesn't cost much.

Samuel

Hey Samuel,

thanks for the suggestion concerning the fader - sounds like a good plan to me.

The TRS insert in reality would probably be a PCB connector to link the PRE PCB to the EQ PCB, I figured they would share audio ground if built in the same box? Although it could be used as a TRS insert point to an external device or the EQ in another box...so I'll take heed of your suggestion.
Trafo-less output sounds good too - i'll add it, thanks.

Any thoughts on the 48v regulator - how does that look?
Cheers Tom

Yes Samuel is right about the fader. You can even take 5kohm fader.
I think about the swing and level that the first amp could be strained to.
You would be more into class A at the highest levels,if you take the 5mA or 10 mA version in that stage as well.
An SSL 4000G has gain-steps at 6 dB levelchange with +/- 18 V supply on the rails. So your 2,5 dB at +/-22 V is really "fineadjustments"
Cheers Bo

Hey sorry Bo, I didn't catch your last post before xmas and forgot about this for a while.

So you reckon I can drop the fader to 5k...thats a good point as the B210 will drive that no probs.

I'm not understanding - are you saying that the first amp will be strained as it is? I think its providing the same amount of gain as it would in the original so it should be ok, no?

I think the gain steps will be 5dB now as I had it. Will try and add the revisions and post a new schem this week or next.

-Tom

Hey folks, finally got some time now so here is the next revision. This one has all changes you guys suggested.

CA61 Rev4

I'm wondering about the the impedance balanced output as the polarity flip will not do anything to this output....is it still worth it in your minds?...to do polarity flip at the input would require another expensive 4pole relay so maybe its worth leaving out....

I have a few other questions - should the gain switch be MMB (shorting) or BBM (non-shorting). My mind says it should be non-shorting so the gain won't go up during contact change (it will drop to unity-ish). However a shorting switch will increase gain a few dB as the resistors parallel during the switch. Opinions? Which is least likely to click?

Another question I have is about the local regulation and grounding. If the EQ PCB takes its +/-V feed from the output of the preamp local regs (so the regs power the pre and eq in parallel) what do I do with my power and analogue grounds on the EQ PCB? Where should everything join up?

Cheers Tom

I'm wondering about the the impedance balanced output as the polarity flip will not do anything to this output...

Click to expand...

One way to solve things would be to place it at the input transformer's secondary. I suspect that this will result in a rather substantial bump when flipping polarity. And the transformer may show reduced performance with flipped polarity, but I don't know these Sowters.

I'd put it in front of the input transformer, and for the mic input only. I tend to use the polarity button while recording, not afterwards.

I have a few other questions - should the gain switch be MMB (shorting) or BBM (non-shorting).

Click to expand...

A few dB increase in gain sounds like a lot less clicking than going from 40 dB to 0 dB and back to 45 dB. At least I have yet to build a mic pre with a BBM.

Where should everything join up?

Click to expand...

Are the different PCBs joined by connectors or by wire (depending on space etc.)?

Samuel

Thanks Samuel, yeah I like the idea of putting the polarity before the input TX on mic only, thats a solution that hadn't crossed my mind. Like you I rarely touch polrev when mixing and if needed its easily done in the DAW or on most consoles if used as a line in device....so I'll do that.

Thanks for clarification on the gain switch - I figured as much but wanted to check as I kept flip-flopping on that decision. Shorting it is.

In terms of different PCBs, they would likely be joined by wire from terminal blocks on each board when racked in the same case. I'm planning to build the two in a 1U box as a strip so they would share power, PCBs connected with wire...

However I envisage a few people might like to build EQ only or add the EQs at a later date and hence put them in different chassis. If thats the case the EQ will need a separate regulated PSU (JLM?) and will be unbalanced I/O and its grounding would need to be dealt with like a separate unit. In this case they would insert into the pre via the TRS in the schematic, so I added your RF protection on the shield there.

Wot do you reckon?

Cheers Tom

In this case they would insert into the pre via the TRS in the schematic, so I added your RF protection on the shield there.

Click to expand...

Ah, that 1k/1 nF I wondered about was my idea :grin: . The 1k needs to be in series with the return connection, sleeve goes stright to chassis. The 1 nF now goes after the 1k to chassis. At high frequencies, the cap looks like a short and directs signals to chassis instead of letting things pass into our PCB. Makes sense now?

About grounding: personally I design all of my PCBs with a dual ground plane flooding every free inch on the PCB (see e.g. PCB.pdf or OpAmp_Test_Jig_PCB.pdf). Though I do not pretend to have years of PCB-design experience I believe that doing so is very good practice and will allow you to just provide one ground-connection per PCB. Add one central ground bolt at your chassis and connect every PCB-ground there ("Pin 1s" go locally to chassis, not to the grounding bolt).

One point I'm not sure myself: Do C11/C12 better go to chassis instead of signal ground?

Samuel

Aha! Thanks, what a schoolboy error - that explanation made more sense than the last one...

Any thoughts on the power grounding?

Cheers again and again
-T

You're faster with typing than I am with editing...

About the shorting switch: I think with a shorting one there is no gain change at all during switching (except what we want, of course)--i.e. from 20 dB to 25 dB: first contact 2 is closed, gives 20 dB. In the middel both 2 and 3 are closed, that makes 25 dB gain. Then 2 is released, makes still 25 dB. Does not mean that there is no click, of course... But if C2 has low leakage, things should be acceptable.

Samuel