ruffrecords

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #20 on: October 15, 2016, 04:18:56 PM »
So, I'll try this again. I have my potentiometer set and can now adjust gain. Thanks Ian for that. I have now replaced cap's to compensate for a better bass response. Apparently distortion is going to be a problem with this circuit even at low levels. How can I compensate for that and eliminate it. Would adding a STMicroelectronics STX 13005 power transistor and a Crimson 4000B output transformer reduce the distortion? Would a better 2nd gain stage based around a few more transistors or even an IC help with this? Or would I be better off adding a 2nd gain stage based around a few transistors (or an IC) and the output transformer?

As PRR pointed out, there is plenty of gain in this circuit so distortion will not be a problem. It is very similar to a circuit I used in a small mixer for location recording back in the 70s. You now have a basic gain block and you can follow it with a fader and then you need to add an output stage to make it into a complete mic pre.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #21 on: October 15, 2016, 05:14:32 PM »
Here's more advice.

Go BUY "The Art of Electronics" 3rd ed by Horowitz & Hill and start reading it.  They'll be lots you don't understand but there will be lots that you will .. including the explanation for simple circuits like yours.

And you will still be referring to it when you are a Guru in your old age.

It should tell you how to add your STM STX 13005 power transistor and the Crimson Audio 4000B output transformer etc.  But don't expect this to reduce distortion etc. to an "acceptable" level without a LOT more understanding.

Also download LTspice and start learning to use it.

I'm going to side with JohnRoberts and PRR on this.  Us oldies have done a lot of design(s) like this and while "good" performance is possible, its not simple.  Ruffrecords is ex-Neve so his idea of "acceptable" is slightly easier.

(Sorry Ian.  Ex-Calrec can't resist a dig at ex-Neve  8) )

If you just want to play with something on a breadboard, play away .. but the breadboard, TAOE and LTspice will teach you a lot more and more quickly than anything we could tell you in a dozen long posts.

But have a look at RIAA preamps in old HiFi amps if you want tested circuits of this sort.  Variable gain which is important for Mike Preamps is harder to do nicely.

I've just looked back at your original post.  Ian's reply is the most appropriate.  It just answers your question.  Curing your circuit's other ills and turning it into a practical mike preamp will take a lot longer.
« Last Edit: October 15, 2016, 05:34:35 PM by ricardo »

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #22 on: October 15, 2016, 05:32:48 PM »
As PRR pointed out, there is plenty of gain in this circuit so distortion will not be a problem. It is very similar to a circuit I used in a small mixer for location recording back in the 70s. You now have a basic gain block and you can follow it with a fader and then you need to add an output stage to make it into a complete mic pre.

Cheers

Ian

Ian, would it be plausible to use (for an output stage) 1k resistors feeding a STMicroelectronics STX13005 power transistor that feeds the Crimson 4000B Transformer into a Amphenol 1/4" connector? If this doesn't seem plausible, could you provide a simple output stage that would work on a single power supply. I can skimp on the transformer if need be. Or, if need be I can spend a little more to get a decent linear power supply so I can use a dual supply op amp to make a balanced line driver. But, I'd like to not do so if I can. I know the 1073 doesn't use linear power. So, I know I can build a decent single supply output stage that connects via a 1/4" jack.
« Last Edit: October 15, 2016, 05:51:36 PM by Leek »

ruffrecords

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #23 on: October 15, 2016, 05:56:46 PM »
Ian, would it be plausible to use (for an output stage) 1k resistors feeding a STMicroelectronics STX13005 power transistor that feeds the Crimson 4000B Transformer into a Amphenol 1/4" connector?

Probably not. The transformer will probably be OK, but the manufacturers specs contain virtually zero design information so it is difficult to be certain. However, if you feed it into a typical 10K input it should be OK but its headroom will be limited.

As for the STX13005, I am not sure why you chose that; it is meant for fast  switching applications and it comes in a TO92 package so it is not really suitable for an audio output stage. I think you would be better off with something like a BD131 on a small heatsink. Even then it does not really have enough current gain on its own to act as an output stage.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #24 on: October 15, 2016, 06:03:51 PM »
Probably not. The transformer will probably be OK, but the manufacturers specs contain virtually zero design information so it is difficult to be certain. However, if you feed it into a typical 10K input it should be OK but its headroom will be limited.

As for the STX13005, I am not sure why you chose that; it is meant for fast  switching applications and it comes in a TO92 package so it is not really suitable for an audio output stage. I think you would be better off with something like a BD131 on a small heatsink. Even then it does not really have enough current gain on its own to act as an output stage.

Cheers

Ian

I just had a STX transistor laying around. So, was trying to figure out how I could use it. What would be the most simplistic way I could use a Crimson 4000B transformer (or heck how about a Triad TY-250P as I have a bunch laying around) to make a decent output stage that connects to a 1/4" jack. I plan to feed the output stage into the line input of my 002. If I'm not mistaken it is a 10k input as most modern interfaces. would it be as simple as connecting the the output and ground from the gain block to the transformer, then connect that to the 1/4" jack?

And if using the transformer is too complicated, a transformerless output stage would be fine. As long as it was single supply. I've upgraded my psu to 18v as well for the added head room. So, any ideas Ian? I was thinking maybe I could either use the transformers or use some cheap single supply op amps to create an electronically balanced output stage.
« Last Edit: October 15, 2016, 06:11:59 PM by Leek »

ruffrecords

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #25 on: October 15, 2016, 06:32:33 PM »
I just had a STX transistor laying around. So, was trying to figure out how I could use it. What would be the most simplistic way I could use a Crimson 4000B transformer (or heck how about a Triad TY-250P as I have a bunch laying around) to make a decent output stage that connects to a 1/4" jack. I plan to feed the output stage into the line input of my 002. If I'm not mistaken it is a 10k input as most modern interfaces. would it be as simple as connecting the the output and ground from the gain block to the transformer, then connect that to the 1/4" jack?

And if using the transformer is too complicated, a transformerless output stage would be fine. As long as it was single supply. I've upgraded my psu to 18v as well for the added head room. So, any ideas Ian? I was thinking maybe I could either use the transformers or use some cheap single supply op amps to create an electronically balanced output stage.

Just to get it going and hear it through your 002 all you need is a simple emitter follower to buffer the  existing mic pre. The output of your BC557 is already at about half the supply voltage so get rid of the 100n output cap and connect the base of another NPN to the collector of the BC557. Use the STX if you wish. Connect its collector to the +ve supply, Connect its emitter to ground via a 1K resistor. It should then draw about 9mA. Connect the +ve of a 47uF electrolytic to the STX emitter and connect the other end of the 47uF to 0V via a 10K resistor. Your mic pre output is across the 10K resistor. it is unbalanced, quite low impedance and should drive you 002 line in no problem.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #26 on: October 15, 2016, 07:07:24 PM »
Just to get it going and hear it through your 002 all you need is a simple emitter follower to buffer the  existing mic pre. The output of your BC557 is already at about half the supply voltage so get rid of the 100n output cap and connect the base of another NPN to the collector of the BC557. Use the STX if you wish. Connect its collector to the +ve supply, Connect its emitter to ground via a 1K resistor. It should then draw about 9mA. Connect the +ve of a 47uF electrolytic to the STX emitter and connect the other end of the 47uF to 0V via a 10K resistor. Your mic pre output is across the 10K resistor. it is unbalanced, quite low impedance and should drive you 002 line in no problem.

Cheers

Thank you Ian. Am I correct in saying that the + of a XLR connects to the Input, the - to the bottom input of the circuit and the final prong is grounded? Also, the 1/4" output has the output from circuit connected to the amphenol connector tip and then the ring is grounded?

Ian

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #27 on: October 15, 2016, 10:32:26 PM »
Ian, I am having some trouble wrapping my mind around connecting a few things. I was wondering, would it be ok to connect R11 of the line receiver to the - input of the preamp stage and then the - output of the preamp stage to R2 of the line driver? So that I have the line receiver feeding both inputs of the mic preamp and both outputs of the preamp feeding the single input of the line driver? I have provided a picture of my schematic and my small build. Perhaps you can help a little more!

https://s12.postimg.org/gb64e1zbh/Mic_Preamp.png

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #28 on: October 16, 2016, 02:10:27 AM »
The correct connection is:

Balanced receiver output to mic input +, and mic input - to ground.
Mic output + to balanced driver input, and mic output - to ground.








ruffrecords

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #29 on: October 16, 2016, 03:39:09 AM »
Thank you Ian. Am I correct in saying that the + of a XLR connects to the Input, the - to the bottom input of the circuit and the final prong is grounded? Also, the 1/4" output has the output from circuit connected to the amphenol connector tip and then the ring is grounded?

Basically yes. You mic pre at the moment is unbalanced in and unbalanced out. For testing you should connect pin1 and pin3 of the input XLR to 0V and pin2 to the mic pre input. The output of the mic pre goes to the tip of the TRS and the ring and screen should be connected to 0V.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'


Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #30 on: October 17, 2016, 12:55:22 PM »
OK.  Unlike John Roberts, I've succumbed to the Dark Side.  Sometimes, the temptation to do something really dirty is irresistible.   :o

In da old days, this type of design would have been done with a Radford LDO & Distortion  Analyser.  This is just an exercise in LTspice .. which you can download.

It's nearly early 1970s SOTA ... but not quite.  That would require an extra Emitter Follower.  But if you are making something like this, why try to make it sound as clean as a 1980s 5534 design?   8)

I was first inspired to do this stuff when Angus McKenzie published a Studio Sound article on improving a Revox A77.  Its mike preamps weren't its best feature.

It's loosely based on LNprimer.doc in my Yahoo MicBuilders Files directory which IS SOTA for what it is supposed to do.  You have to join.

Read it to understand some of the Low Noise design process ... and "The Art of Electronics".
« Last Edit: November 18, 2016, 10:22:18 PM by ricardo »

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #31 on: October 25, 2016, 11:06:05 PM »
If you are just feeding the Line I/P of your Digi 002 (simulated by R9), you can use somewhat less than 30V for the supply.  Adjust R2 for symmetrical clipping if you do this.  Presently, it will clip just before 16dBV.  The bias chain R1 & R2 is decoupled by C5 to get rid of noise from the power supply line before it reaches the sensitive input.

It’s unclear what input is required for FS on the Digi 002 line inputs.  The most relevant number seems to be 18dBu which is 6.15V or 15.8dBV  This needs the 30V supply

Gain is set by switching R6 to give 4 gains between 42dB(sorta) & 12.1dB.  You could use a Reverse Log pot but that would be yucky and unacceptable to the BBC 

R10, 11 & C4 implement an Impedance Balance O/P stage.  Output resistance, Ro, will vary with the gain setting so R10 might need tweaking for a good CMRR compromise.

If the preamp is next to your Digi 002, you can probably use this as drawn.  If it is more than a couple of metres, replace R10, 11 & C4 with your Crimson 4000B 1:1 transformer ... giving better balance and adding its distortion to the vintage THD profile characteristic of the 2 transistor amp.
http://www.crimsonaudiotransformers.com//FileStream.aspx?FileID=2

The Crimson spec looks good BUT there is no design impedance or winding resistance specified.  Both frequency response & distortion are VERY dependent on this.  If it was designed for a low impedance, using a higher impedance at both input & output will degrade LF response & THD.  I’m going to assume it is designed for 600R or less on at least one side.  Our worst case is Ro = 188 (when R6 = 82) so we should get better than the specified +/- 0.5dB frequency response 20Hz – 20kHz provided the DC resistance of the windings is small enough not to take the total ‘source’ resistance above the design resistance.

'Professional' gear would have  a Sowter microphone input transformer and make this 'acceptable' to the BBC in 1970.  I’ve shown Lundahl & Jensen equivalents.  They add another 14dB gain and this would actually have very good noise performance even today.  Rest of document doesn’t include input transformer gain/THD

These transformers are 1:5 so their impedance transformation will be (1:5)^2 = 25.  A 150R dynamic mike will present 3K8 to the amplifier (simulated by R12) .. which gives near optimum noise performance for the input transistor Q1, BC548C running at 94uA.

The load seen by the mike is R7 transformed by the transformer; 47k / 25 = 1k88

If your dynamic mike is within a couple of metres, you could probably feed it unbalanced into the preamp without the expensive input transformer.  But P48V would require it … noise would be better … and it gives 14dB of clean gain.  See the Jensen application notes for details.
« Last Edit: October 31, 2016, 09:50:56 PM by ricardo »

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #32 on: October 25, 2016, 11:16:19 PM »
Hey Ricardo, I've actually changed things and went with an IC front end followed by a transistor emitter follower output stage. It seemed a lot simpler and I've been debugging it. Here is the schematic. I have also provided two different BC 547 emitter follower output stages. Though, I'm leaning towards the first/top one.

https://postimg.org/image/9h3b2wznt/

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #33 on: October 25, 2016, 11:36:03 PM »
The only sensible reason for building something like this today is that the vintage circuit provides some euphonic (nice sounding) distortion.

Worst distortion case with gain set to 42dB.  THD is 1% at 5V output and mainly 2nd (h2).  Clips at about 5.7V But the onset of clipping is quite soft.  You could push it a bit harder .. it would distort and still not sound too harsh. 

This depends on the music.  Good small unaccompanied choirs or piano need really clean preamps with huge headroom not to sound nasty.  Other stuff with big dynamic range, like drums, actually don’t sound nasty or even different if clipped.

Input frequency is 1kHz.  THD doesn’t change much with frequency at any level below clipping.
« Last Edit: October 25, 2016, 11:39:14 PM by ricardo »

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #34 on: October 25, 2016, 11:42:58 PM »
At 12.1dB gain, there is a lot more feedback and the clipping point is moved to 6.3V  THD again mostly 2nd but the clipping is more hard edged.  Still softer than a 1980s design.

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #35 on: October 25, 2016, 11:52:18 PM »
Hey Ricardo, I've actually changed things and went with an IC front end followed by a transistor emitter follower output stage. It seemed a lot simpler and I've been debugging it. Here is the schematic.

https://postimg.org/image/9h3b2wznt/
What PRR was trying to tell you  in the other thread is that all the 5532 circuits you have posted have terrible noise and would be unacceptable as mike preamps.

If you want to use an IC, THAT 1510/12 are probably best.  They have excellent datasheets & application notes.  If you build them correctly to their spec, you would have something that sounded like your Digi 002 preamps.

What I've posted is something a top 1970's mixer designer at eg N**e would have done.  If you use the input transformers, it's noise performance would be difficult to fault.  It will have the 'sound' of the famous name mixing desks of that period.

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #36 on: October 26, 2016, 12:25:46 AM »
So, what you're telling me is this preamp build would be ok for things like drums, electric guitars and louder sources where distortion and things like would be ok? I'm not looking for a huge super clean preamp. I'm just looking to build a working preamp. Upon learning how to get this circuit going, I can add Jensen/Lundahl transformers later to increase headroom and noise floor. Upon looking over my schematic ricardo, how does it look? Will it work?

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #37 on: October 26, 2016, 03:33:13 AM »
So, what you're telling me is this preamp build would be ok for things like drums, electric guitars and louder sources where distortion and things like would be ok?
Something like my circuit would have been used for everything by the top studios in the early 1970s.

If you are recording certain types of high dynamic range stuff like drums, it would still do an excellent job.  For unaccompanied choir and piano, good 'modern' IC stuff would give better results.  You need to distinguish between stuff with high dynamic range (soft bits & loud bits) and stuff which is just loud.  The former is MUCH more difficult to record.

Quote
I'm not looking for a huge super clean preamp. I'm just looking to build a working preamp. Upon learning how to get this circuit going, I can add Jensen/Lundahl transformers later to increase headroom and noise floor.
If you want a working preamp, choose one of the simpler circuits like ESP or the THAT datasheets and build it EXACTLY as shown.

If you want to understand how to design good LN amps, you will learn more building my circuit and working out why it is better than your earlier efforts.

I've recommended you build it exactly as shown and add transformers later.  I've also told you what you gain when you add the output Crimson and the input Sowter.

To get the full advantage, you need to do your homework ... reading "The Art of Electronics" or my LNprimer from Yahoo MicBuilders.

Leek

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #38 on: October 26, 2016, 08:27:05 PM »
Ricardo, with my schematic that I've provided and Ian helped me add a pot and unbalanced output too, could I not just add a 10468 style input and 11148 output transformer to balance the INS and OUTS? It seems redundant to now throw my components away and just buy new passives to build your styled preamp.. Ian said it has plenty of gain so distortion (noise) wouldn't be a problem..

If you'll look at it, its a BC547 input, feeding a BC557 that then feeds another BC547 output stage. I could use the 10468 style transformer at input to balance the signal into the BC547. I could then use the last BC547 in a emitter follower stage to feed the 11148 output transformer, correct? Is the 2N3055 more suitable for the output stage with the 11148 style transformer?
« Last Edit: October 26, 2016, 08:31:48 PM by Leek »

Re: Mic Preamp. Adding things and getting ideas. Would love help please.
« Reply #39 on: October 26, 2016, 09:21:16 PM »
Ricardo, with my schematic that I've provided and Ian helped me add a pot and unbalanced output too, could I not just add a 10468 style input and 11148 output transformer to balance the INS and OUTS?

How about you post links to 10468 & 11148?  You make it difficult for people to understand you .. poor pics on some other website etc.  ...  the  less likely you will get good answers

Quote
It seems redundant to now throw my components away and just buy new passives to build your styled preamp.. Ian said it has plenty of gain so distortion (noise) wouldn't be a problem..

If you'll look at it, its a BC547 input, feeding a BC557 that then feeds another BC547 output stage. I could use the 10468 style transformer at input to balance the signal into the BC547. I could then use the last BC547 in a emitter follower stage to feed the 11148 output transformer, correct? Is the 2N3055 more suitable for the output stage with the 11148 style transformer?
New passives about $0.02 / each for the resistors.

Why don't you build it as "Ian suggested" (I don't think you understand Ian fully) and then build my version.

The most expensive part is the 30V supply but this is needed to drive your Digi 002 to the spec I show.  I'm not sure Ian has worked out if your version can drive the Digi 002.

You haven't shown anything for us to look at.