Transformerless 990 micpre

Hi there,
I wonder how will 990 DOA work in transformerless circuit? Need some help.
I took J Hardy's M-1 schematic [http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf]and mixed it a bit with a 312 because I don't want servo in this preamp I'm building. Transformerless output section is from Jensen AS017 [http://www.jensen-transformers.com/as/as017.pdf]. Not shure how it will work  :-\ Hardy's M-1 haves 150R:600R 1:2 transformer on input and 600:600 1:1 on output. Don't have any clue what to put in front of 990 DOA and how to tweak AS017 output to get best out of DOA?

this is progress so far


any ideas?
thanks

uncle said:

Hi there,
I wonder how will 990 DOA work in transformerless circuit? Need some help.
I took J Hardy's M-1 schematic [http://www.johnhardyco.com/pdf/M1_M2_M1p_20031025.pdf]and mixed it a bit with a 312 because I don't want servo in this preamp I'm building. Transformerless output section is from Jensen AS017 [http://www.jensen-transformers.com/as/as017.pdf]. Not shure how it will work  :-\ Hardy's M-1 haves 150R:600R 1:2 transformer on input and 600:600 1:1 on output. Don't have any clue what to put in front of 990 DOA and how to tweak AS017 output to get best out of DOA?

this is progress so far

any ideas?
thanks

Click to expand...

Essentially, there's a contradiction between using a 990 an ruining its performance by running it unbalanced. Unfortunately, there's no way to use the 990 (or any other opamp) as a good balanced mic pre, other than putting a good (read expensive) xfmr in front of it. You could also think of putting a long tailed pair in front of it, but the 990 is definitely overkill in that application (and probably oscillation prone). As far as the output scheme, it's the very common balanced-impedance type, it works well, doesn't have the problems associated with cross-currents, it has one disadvantage: in conjunction with an unbalanced input, if you have the wrong connection, there's no signal at all. On the other hand, this turns out to be an advantage, because you don't risk to be out-of-phase.

If budget is an issue and if I were you, I'd spend the money on the transformers and ditch the 990/2520 DOA.  There are modern chips in PDIP8 packages that you can use instead. 

Then, when you have some extra $$$ and still want a DOA, then buy a DOA and substitute it for the PDIP8.

But you may be surprised at what a modern PDIP8 chip sounds like compared to a DOA at only 1/20th the cost of a DOA!

> 990 DOA work in transformerless circuit?

Better than my old Yamaha. With the same restrictions: unbalanced input, no Phantom power.

Alternatives are: use two opamps (and suffer some noise increase), use a true diff-amp (in audio we call these "transformerless mike amps"), or use a transformer as Deane meant you to do.

For general modern studio work, as Owel says: sell the lovely 990 to buy a decent tranny and a $5 chip.

http://ingramengineering.net/white-paper-why-transformer.php

There is no problem to use a discrete opamp in a transformerless preamp if you're willing to add a discrete transistor frontend. Check the Green Pre and the various alternative implementations in the Meta--replacing the IC opamps with the 990 should be straightforward in most cases.

Samuel

Thanks for help, guys!

PRR said:

.....
Alternatives are: use two opamps (and suffer some noise increase),
.....

Click to expand...

Two opamps like this?


I borrowed input from 9k pre. Will try to figure out how to wire input transistor in, as adviced by Samuel. I don't understaint 9k and Green schematic fully. Some guidance will help

no put the gain pot between the two - inputs.

There are tons of examples.. consider searching

JR

If you'll be using (2) 990 in your pre design, you might as well buy an input transformer. 
Your total $$$ outlay will be about the same. ... and you'll get the benefits of using an input trafo.

Owel,
I'm going to use diy 990. Anyhow, i'd like to build transformerless DOA micpre and share PCB so can everyone build one. Maybe with 2520 or 1731 or SOA-1, doesent matter. Shure, it's not going to sound like it's transformer brother, but it'll have it's own sound. And it's going to be cheap as well

thanks everyone for help
cheers

JohnRoberts said:

no put the gain pot between the two - inputs.

Click to expand...

Think I figured out input on green pre.. Give me a few minutes to redraw and post new circuit

Am I getting closer? 

You're getting closer. From a quick glance you are still lacking the typical current sources used to provide operating current for the input devices. I guess the Rs and Cs connected oddly to the input bases, may be intended for emitters. As drawn the opamps will adjust their output operating point to provide that current through the 10K feedback resistors (I have actually done this before on purpose to reduce active parts count and/or noise gain), in your design that means the output blocking capacitors are backwards and you will give up output swing.

In general, with this topology a lot of the heavy lifting (noise and gain) is being done by the front end devices so using high performance DOA inside the overall loop is perhaps overkill. If compelled to go discrete there I would be tempted to make a 3 input/2 output compound opamp to reduces parts and complexity, since the input stage of those internal DOA are operating relatively fixed.  Far easier (and probably cheaper) to drop on some decent IC opamps there.

This topology was addressed in Sam's preamp thread, and has been discussed around. Perhaps search "Graeme Cohen" who has generally been credited with this topology while it or variants have been in use prior to his publication AFAIK. 

JR

As far as I've seen this should work (R10/R11 bias the input transistors), but IMO there are a few nonidealities in the Green Pre design which are now reflected in your schematic. I suggest you use A_r1.pdf as starting point; you don't need to change anything except replacing the ICs with the discrete opamps and skipping the 100 nF bypass caps (on-board on the 990). If you want +/-24 V rails I can point you to the necessary changes.

Samuel

the LM394 used in je-990 has lower noise voltage than the BC550C used in the front-end.
je-990 has less than 1nV/rtHz.

Pier Paolo

JE-990 has less than 1 nV/rtHz.

Click to expand...

No, the specification is misleading. Actual value is slightly above 1.1 nV/sqrt(Hz). But the BC550 is indeed a poor choice for a transformerless mic pre frontend (what I partially said before). The 2N4403 I used in the schematic linked above is even better than the LM394.

Samuel

You are right Samuel but I've got an error ,I wanted to write "LM394 has less than 1nV/rtHz" but I've write "je-990 has less than 1nV/rtHz" .

Infact , when the transistors inside this IC are used instead the bc550c their voltage noise is under 1nV/rt Hz......having each transistor 1.23 mA of Ic. 

using LM394 in the front-end could be possible to eliminate c17 from schematic , but with 2n4403 the voltage noise should be lower. 

I've not tried to eliminate c17 from schematic using LM394 but should be right doing it because these transistors inside LM394 are very similar so the currents in their collectors are (quasi) equal so the current in the pot should be near to zero. 

Samuel Groner said:

As far as I've seen this should work (R10/R11 bias the input transistors), but IMO there are a few nonidealities in the Green Pre design which are now reflected in your schematic. I suggest you use A_r1.pdf as starting point; you don't need to change anything except replacing the ICs with the discrete opamps and skipping the 100 nF bypass caps (on-board on the 990). If you want +/-24 V rails I can point you to the necessary changes.

Samuel

Click to expand...

Unless we're looking at different schematics R10 and R11 are collector loads.  The schematic you linked too also provides input device current from the feedback resistors but in your link the input devices are biased -5V or so. This schematic looks like inputs are biased at 0V so opamp output will need to sit at negative voltage to make required current.

This will mostly work if output caps are turned around, but i still suspect some parts may be missing or overlooked.

I though this topology had been discussed to death but apparently not.

JR

I've the sensation that this schematic is not so stable.....................