Discret Mic Preamp with 1:5 transformer

[Lemontheo]

Hi,
My first tread here. I'm quite familiar with electronic in guitar pedals and tube amps, but totally newbie in Studio electronic.

A couple months ago, i've bought 14 Beyerdynamics transformers (15€ all !!!) with 1:5 ratio (impedance > 200R:5k)
I'm gonna use 1 or 2 to make DI boxes (Bo Hansen schematic).

With the others, I plan to make a 8 way simple preamp to do recording (rock band ).
Each output will be plugged into a line input of a USB sound card.

I have 2 questions:
- Is an output transformer necessary in this configuration?
- Does anyone have a diagram that might match?

Best regards

Théophile (from France)

 

[Dylan W]

I have 2 questions:
- Is an output transformer necessary in this configuration?

No. The simplest way omitting an output transformer is something called “impedance balanced.” Basically a resistor in series with each leg of the signal. Most designs will also want a dc-blocking cap or dc servo. Cap is simpler and more rock and roll.

- Does anyone have a diagram that might match?

Best regards

Théophile (from France)

View attachment 116101

I would start with something like an API 312 schematic, which is very simple: input tx -> non-inverting discrete op amp -> output tx. Omit the output transformer and look up how to make the aforementioned impedance-balanced output. As far as the amplification stage, you can use lots of things. 2520 is an obvious choice. I like the BigFetBloke, a simple design put together by a member here years ago. Low part count, high current, great sound. Something like a cleaner Neve.

Each type of opamp has its own ideal transformer ratio for best noise performance (2520 is supposed to be around 1:7, 990 1:2) but 1:5 should be good enough for just about everything as long as you aren’t distance miking harpsichord with a ribbon.

 

[gyraf]

..or the focusrite isa-110 input stage - a NE5534, a gainswitch, and an input transformer

e.g. the first part here: Focusrite

as Dylan says, go impedance balanced. If it's good enough for Neumann and GML...

/Jakob E.

 

[L´Andratté]

This is usually my jack-out-of-the-box moment(*) to point out this again: Ricardo´s mic pre
Not gonna say more than I built a bunch with 1:5 transformers and it sounds absolutely awesome.
As in "it has a sound", if that´s what you´re after and it´s so simple you can built it up short time and try.

(*) because it feels like it deserves more praise (;

 

[Lemontheo]

No. The simplest way omitting an output transformer is something called “impedance balanced.” Basically a resistor in series with each leg of the signal. Most designs will also want a dc-blocking cap or dc servo. Cap is simpler and more rock and roll.

Thank you. I'm definitively going to do that! (the Rock'n'Roll way! )

I would start with something like an API 312 schematic

Yes, I have seen this schematic.

I like the BigFetBloke, a simple design put together by a member here years ago. Low part count, high current, great sound. Something like a cleaner Neve.

It's a discret opamp to, like the 2520. I've found some schematics, but all of them are different. And my goal is to have a preamp with a color. I don't want a clean preamp. Not sure this one would be the good one for this task.

but 1:5 should be good enough for just about everything as long as you aren’t distance miking harpsichord with a ribbon

Good to know! Thanks a lot

..or the focusrite isa-110 input stage - a NE5534, a gainswitch, and an input transformer

As i said, I'd like to make a preamp with a colour. Is the Focurite colorful?

This is usually my jack-out-of-the-box moment(*) to point out this again: Ricardo´s mic pre
Not gonna say more than I built a bunch with 1:5 transformers and it sounds absolutely awesome.

Thanks, I have ever seen this schematic. The Hudson Broadcast Overdrive uses this typology.

Like I said before, I'm new in Studio DIY stuff (and on this awesome website!)
I am a HUGE fan of Led Zeppelin, and I've read that they uses a Helios Mixer desk. So I have take a look to the Helios schematics, and I wonder if the Helios 2128 V3 could be what I'm looking for.
I've made some mods on the original schematic. It is a good way?

 

[L´Andratté]

Nice find! It´s a pretty standard topology of the 60s/70s. It´s even in The Art Of Electronics (4thed.). The biasing on the Hudson Broadcast is very different though, the mic pre is not an overdrive, it can be pretty clean and low noise, especially when buffered.

 

[Lemontheo]

I'm gonna try it.
The gain is controlled by R6?

 

[L´Andratté]

Right. It controls how much AC feedback is shunted to GND.

 

[Dylan W]

It's a discret opamp to, like the 2520. I've found some schematics, but all of them are different. And my goal is to have a preamp with a color. I don't want a clean preamp. Not sure this one would be the good one for this task.

It’s not clean exactly, it’s just fat and fast. Still has a distinct character. It’s current feedback, like the Neve and Helios. Sounds really good for drums and vocals. A lot of the character of whatever you build will come from the transformers though.

Good to know! Thanks a lot

As i said, I'd like to make a preamp with a colour. Is the Focurite colorful?

I considered recommending the Focusrite design, it’s nice, but it’s pretty clean sounding, especially without the special output transformer which has a feedback winding. Otherwise it’s just the input transformer and a 5534 op amp.

Thanks, I have ever seen this schematic. The Hudson Broadcast Overdrive uses this typology.

Like I said before, I'm new in Studio DIY stuff (and on this awesome website!)
I am a HUGE fan of Led Zeppelin, and I've read that they uses a Helios Mixer desk. So I have take a look to the Helios schematics, and I wonder if the Helios 2128 V3 could be what I'm looking for.
I've made some mods on the original schematic. It is a good way?

For what you want Helios is probably a great choice. They definitely have lots of character. The vintage ones didn’t use output transformers anyway, and they used the same Beyer transformers (though 1:10 rather than 1:5) so you will be quite close out of the starting gate.

I would scrap the filter section and emitter follower and build the line amp as your output stage

 

[Lemontheo]

Thanks for your advices!
So I'm definitely dropping opamps!

I'm going to do PCBs of the Helios preamp. (and I'm also going to try the Ricardo's Mic Preamp)

I would scrap the filter section and emitter follower and build the line amp as your output stage

When you say "build the line amp", you mean a second block after the input section?

In the early version, there is a 2128 (or 15C1) at the input, the EQ, and then an other 2128 before the output transformer.
Is the "newer" version with the 22113 at the output better than the earlier?

 

[MisterCMRR]

An application note "Interconnection of Balanced and Unbalanced Equipment - Jensen AN003" I wrote about 30 years ago explains how to convert any single-ended (unbalanced) output to a balanced output in section 2.4. See https://www.jensen-transformers.com/wp-content/uploads/2014/08/an003.pdf. The note contains other information that may be useful, too.

 

[TwentyTrees]

Thanks for your advices!
So I'm definitely dropping opamps!

I'm going to do PCBs of the Helios preamp. (and I'm also going to try the Ricardo's Mic Preamp)

When you say "build the line amp", you mean a second block after the input section?

In the early version, there is a 2128 (or 15C1) at the input, the EQ, and then an other 2128 before the output transformer.
Is the "newer" version with the 22113 at the output better than the earlier?

The 2128 / 15C1 would struggle to drive a line output without an additional buffer - the 22113 is a more robust line amp, so if you're building from scratch I'd look at that.

 

[Lemontheo]

Hi,

how to convert any single-ended (unbalanced) output to a balanced output in section 2.4

Thanks a lot. This document is very helpful!

The 2128 / 15C1 would struggle to drive a line output without an additional buffer - the 22113 is a more robust line amp, so if you're building from scratch I'd look at that.

Ok. Thanks. So i wanna do that!

So I have drawn a schematic of the 15C1 with a phantom switch, a polarity switch, a pad before the transformer, and a balanced output. What do you think?
(the transistors are not the good one. It's just for drawing the schematic)

 

[Dylan W]

The 2128 / 15C1 would struggle to drive a line output without an additional buffer - the 22113 is a more robust line amp, so if you're building from scratch I'd look at that.

Yeah as TwentyTrees says, I would put the 22113 after the input amp. The input amp by itself (as in your latest schematic) should be fine driving a 5-10k interface input if you know you’re always going straight in. The line amp (2 stage design) would give you more drive capacity and probably a bit more mojo, also you could put a pot between stages to give you more control over how you drive it.

 

[Dylan W]

Hi,

Thanks a lot. This document is very helpful!

Ok. Thanks. So i wanna do that!

So I have drawn a schematic of the 15C1 with a phantom switch, a polarity switch, a pad before the transformer, and a balanced output. What do you think?
(the transistors are not the good one. It's just for drawing the schematic)

View attachment 116244

Don’t forget a DC blocking cap in series with the gain pot

 

[Gavin Pursinger]

Distance miking a harpsichord...I love this place !@@!

 

[MidnightArrakis]

Distance miking a harpsichord...I love this place !@@!

[Distance miking a harpsichord] -- I once recorded a "harpsichord concerto" using a ROYER SF-12 stereo ribbon microphone and it turned out terrific!!! Really very, very sweet!!!

Another time, I recorded a "hammer dulcimer" using a RODE NT-4 that was placed about 8-feet up and behind, yet over the performer aimed down at the hammer dulcimer itself. That also turned out to be terrific!!! Extremely clean with a wonderful stereo image!!!

I just thought that I would throw this in.

/

 

[Lemontheo]

Hi,
after a good week of reflection, my project is refined. I browsed the forum (not all of it! ) on topics about Helios, and I came across this super interesting thread.(thanks dogears )
I now know what I am going to do. I'm going to make a 2U rack with 8 complete preamps (without the EQ part, but with an insert jack to connect one later).
Each slice will be distributed over 3 PCBs (one for the controls, one for the active part, one for the connectors).
The power supply will be separate.

I'm attaching the diagram used. I have 2 questions:
- Is this the best place for a Level?
- Have I connected the jack/xlr output correctly?

If you have any other suggestions!

 

[TwentyTrees]

Hi,
after a good week of reflection, my project is refined. I browsed the forum (not all of it! ) on topics about Helios, and I came across this super interesting thread.(thanks dogears )
I now know what I am going to do. I'm going to make a 2U rack with 8 complete preamps (without the EQ part, but with an insert jack to connect one later).
Each slice will be distributed over 3 PCBs (one for the controls, one for the active part, one for the connectors).
The power supply will be separate.

I'm attaching the diagram used. I have 2 questions:
- Is this the best place for a Level?
- Have I connected the jack/xlr output correctly?

If you have any other suggestions!

On quick preview, your pot is where the original gain switch is and is the right place for gain control in this circuit. Likewise, the balanced XLR and unbalanced TS outputs look sound.

One question - can your Beyer input transformers be strapped for a 1:10 ratio, as well as 1:5? The Helios amp is optimised for 1:10, so your noise performance would be worse with 1:5.

[Edit: just found the relevant Beyer input transformer catalogue, looks TR/BV351 105 indicates a 1:5 ratio with a centre-tapped primary. Worth a try!]

 

[Lemontheo]

Hi,

Thank you.

your pot is where the original gain switch is and is the right place for gain control in this circuit.

The level Pot is between the emitter follower and the last stage. Is it good?

One question - can your Beyer input transformers be strapped for a 1:10 ratio, as well as 1:5? The Helios amp is optimised for 1:10, so your noise performance would be worse with 1:5.

[Edit: just found the relevant Beyer input transformer catalogue, looks TR/BV351 105 indicates a 1:5 ratio with a centre-tapped primary. Worth a try!]

Here is the schematic of the Beyer TR/BV351 105

You're right, it worth a try!

(by the way, very impressive work )