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Author Topic: Mic pre recapping and modification  (Read 2312 times)

Offline nicoro

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Mic pre recapping and modification
« on: February 25, 2016, 09:11:48 pm »
Got a sweet deal on a mixer interface with just 4 mic pres but 16 AD 24/96 converters.  This unit can send all 16 channels individually via USB-2 to a compu to record or manipulate.   So need more mic pres to make this fully functional.  Luckely have some late 1980s 8 mic pre 1 rack unit mixers which have one combined output.  Looking over the schematic it appears they can be modified.  Instead of the 8 mic pres combining into a single output each mic pre will have its own output with itīs own volume control.  These units have transformer inputs with its advantages and also disadvantages. The schematic and manual is here:


Obviously these 20-30 year old units are due for an electrolytic recap which should include the phantom voltage, preamp, and power supply electrolytics.  Was thinking of using Panasonic FM not bypassed as coupling? The power supply uses simple on board noisy 7812, 7912 regulators and a wall wart 27 volt transformer which produces a + /- 12 volts.  Several improvements could be made in the Power supply!  Could increase transformer voltage to 40+ Volts and use LM317/LM337 adjustible regulators for a +/_  15-20 Volts with less noise and more headroom?  The SB737 transistor and 5534 chip are listed as able to handle that voltage

The preamp design itself is a little quirky.  In the schematic the input transformer is placed BEFORE the phantom voltage and  pad, etc this then requires lytic caps 10UF/50v Cap 109 and 111 to be in the signal path to absorb DC unnecessarily? In the Jensen transformer site under Schematics they list a generic  transformer input  where the input transformer comes after the phantom voltage, pad etc thereby avoiding this.  Putting the pad before the input Trans also allows for padding down XS input to avoid saturating the trans. So there then is also the  question of wether the circuit should be modified and the input transformer moved back further in the circuit??  I donīt want to second guess the designers but it seems they were designing more for safety than the ultimate sound quality. But what would be the benefit of modding and increasing the voltage rails to increase overhead if the limit is set by the input trans before the padding??  So moving the input trans after the 30db pad should allow to get rid of the 10UF caps and the 10 ohm resistor and what else?


Other than that the modifications to make this an 8 in 8 out mic preamp seem fairly straight forward.  The output from the 5534 chip via the DC blocking cap C105 22UF is taken to the input leg of the volume control=variable resistor 101.  And itīs middle leg=adjustable output is connected to the tip of the patch jack which will be converted to mic output instead.   Seems simple enough.....an external pad could be used for XS input signals overloading the input trans if left in its present position.  Any advice suggestions appreciated.

Offline Jim Williams

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Re: Mic pre recapping and modification
« Reply #1 on: February 26, 2016, 11:22:18 am »
The preamp is a 1970's style intrumentation amp in front of an opamp. The transformer is an add-on as it's not really needed. The 2SB737 transistors are not in the opamp feedback net so THD will be high.

Quality caps, metal film resistors and opamps could improve it, but that front end really needs a redesign so those transistors are in the feedback loop, ala "trans-amp".

Offline nicoro

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Modding 8 channel mixer to 8 in 8 outs
« Reply #2 on: February 26, 2016, 03:40:25 pm »
Hello Jim:

Thanks for responding.  Yes that front end was designed so that you could bypass the input trans and still have a functioning preamp but makes no sense as a recording mic pre.  Since the units are in Guatemala shipping to the states for mods will be cost prohibitive so mods will have to be local.

Have two of these and would like to KEEP input transformers in one of the units for that "colored" saturated trans sound so will move the transformer inputs to after the 30dB pad and remove the 10UF and 10 ohm resistors as they will no longer be needed.  Anything else that can/should be removed before the SB737 transistors after this?? So far the only lytics caps left in circuit to be used is the phantom cap and the DC blocker after the 5534 chip.  To replace the chip DC blocker cap  was planning to use a Panasonic FM 22UF or greater with WIMA mkp-4 0.1UF PP bypass...any other suggestions?

Power supply mods: Will proceed as in original description with 18-20V rails and LM317/LM337 regulators.  Perhaps will go with a 50+V power transformer use that for the Phantom as well as the preamp volt rails?  Also add an outside phantom swith on each channel...tight but do able.

Those SB737 transistors were low noise for the 80s but later Hitachis/Toshibas etc were better as you have pointed out in other posts (did my homework). The 5534 chip maybe improved upon but always afraid of placing very fast chips in a PCB not tested for that...but easy enough to remove if it causes oscillation etc  What chip do you recommend here and any circuit changes to acomodate it?  Have no idea how to incorporate the SB737 into the feedback circuit...Transamp etc. Is it really worthwhile in the transformer input version.  On the second unit to mod would like to remove/bypass the input trans and go for the trans.  Point me in the right direccion if possible.

Thanks Jim I know itīs a lot


« Last Edit: February 26, 2016, 07:08:20 pm by nicoro »

Offline nicoro

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Re: Mic pre recapping and modification
« Reply #3 on: February 27, 2016, 12:45:16 pm »
Been doing some research into the Transamp mod of the instrumentation front end. 

Jim has covered the conversion in the Soundcraft consoles  600 series etc.  Likely can go with similar changes in this mic pre front end.  For now I will keep the SB737 transistors and go with a more modern chip as suggested by Jim.  Will copy previous discussions on this topic here.

Originally Posted by Jim Williams
It's a rather simple mod, you remove the 22k resistor and 100 pf cap in the opamp's feedback loop. Connect the 22k resistor with a 10 pf cap across it to the transistor's emitter, the transistor that feeds the - or inverting input. Don't connect to the non-inverting (+) input or you create an oscillator! Change the 100 pf cap from the non-inverting input to 10 pf, that open's up the bandwidth to 200k hz. You may need a small phase lag cap across the opamp's inputs, a 220 to 470 pf cap will usually work. That up's the phase margin so the opamp remains stable.
Or, get a ver. 7 or higher 600, the changes are already there. Changing the transistors helps at higher gains, try 2SA1084's or 1316's. The opamp is also very important, many can be used, try the LME49720, AD8599, LM6172, LT1358, etc. Local .1 uf psu bypasses are required for wideband opamps.

MoreNow, bypass the electrolytic caps with Wima MKP-2 .01 uf 250 v. Change C8 to 470 uf with a .1 uf Wima MKP-2 100v bypass cap. Change R10 to 15 ohms to make up the 6 db you lose at the top of the gain scale. Remove RIC 1 and add 4 4.75k 1% metal film resistors. Add a pair of .1 uf mono ceramic caps from pins 4 and 8 to a close by ground. Now you can play with opamps. Try LT1358CN8, AS8599, LME49720NA, LM6172. Check with a scope for stability as the second half runs the filter which is a follower, 6172's don't usually like followers.

Jim Williams
Modifing the front end from an instrumentation design to a "trans-amp" does wonders for THD reduction. Since the mic preamp is also your line input for mixing, cleaning that up does wonders.
Pull off the feedback resistor from the inverting opamp input pin 2 and attach it to the emitter of TR2. Leave the feedback cap in place. Then remove R12/C10 from pin 3, the non-inverting input. Attach those parts from the emitter of TR1 to ground. This applies to versions 1 and 2.
Use 10 ohm 1/2 watt fuse resistors. Use 470 uf/25V bypass caps. Use Wima MKP-4 .1uf 250V for the ground decoupling and chassis ground de-coupling. Add two more for the .1 uf mylar psu decoupling, 4 are used total.

Use 4.75k and 2.21k for the fader amp resistors. Use a 22 pf feedback cap. I use a 100 ohm resistor in place of the 2.2 uf cap, short out the output cap and use a LME49990 opamp, lowest noise available. Then it's direct coupled, super low noise and fast. Then be prepared to hear more "uglys" in your music
 comments here

Pic of mod in the schematic is attached