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Anyone seen this input circuit before? (Panasonic/Ramsa WR-500)
Old 27th June 2015
  #1
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Anyone seen this input circuit before? (Panasonic/Ramsa WR-500)

Hi, so I got this thing on a whim, because I think it looks cool, built into a little suitcase. I've seen Ramsa schematics before and they are all pretty much the same, but this one is a bit different, so i thought I'd share and ask if anyone (Jim W, Matt S, et al) if there is any merit to swapping the 4 transistors on the input (2SC2557TU) for something new. I'm already going to put in some LME49720 for the 4560, but I can't find any datasheet on the transistors, so I figured maybe someone here would know.

Thanks!
Attached Thumbnails
Anyone seen this input circuit before? (Panasonic/Ramsa WR-500)-wr500input.jpg  
Old 29th June 2015
  #2
Standard instrumentation amp with a pair of discretes in front of the opamp, a 1970's design. It will have high THD at the top of the gain pot rotation. Re-designing it is a lot of work, probably not really worth it after all, it's Ramsa. Stripping it all down, adding parts to the rear and cutting traces would be involved. Those circuit boards are poorly made and may not survive the surgery.
Old 2nd July 2015
  #3
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Quote:
Originally Posted by weary View Post
Hi, so I got this thing on a whim, because I think it looks cool, built into a little suitcase. I've seen Ramsa schematics before and they are all pretty much the same, but this one is a bit different, so i thought I'd share and ask if anyone (Jim W, Matt S, et al) if there is any merit to swapping the 4 transistors on the input (2SC2557TU) for something new. I'm already going to put in some LME49720 for the 4560, but I can't find any datasheet on the transistors, so I figured maybe someone here would know.

Thanks!
BC547CBU will work. I would buy a few extras to make sure they match. BTW most semiconductor mic preamps are like this one in one shape or form.
BTW I would change c1,c2 and c20 to47uf since your going that far

Last edited by drtechno; 2nd July 2015 at 04:22 AM..
Old 2nd July 2015
  #4
BC parts are noisy, their lowest noise NPN is the BC550. Toshiba 2SC3329BL are less noise = .7 nv/hz/sq.

Since the transistor pairs are not within the corrective feedback loop of the opamp, THD will be high. Placing them inside the feedback look al la 'trans-amp' would fix that.
Old 3rd July 2015
  #5
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I've never had a problem with bc transistors. the basic design is not bad if they actually used a split rail power supply on the op amps like everyone else does.
most of the noise would be from the resistors and not from the active devices. however I do see there could be a way to inject some nfb into the the emitter of the one side of the first stage from the output of ic2 to get rid of some more noise.

but I can see that the quality of the power supply would be quite paramount in this design.
Old 3rd July 2015
  #6
I first learned about the sonic problems of BC transistors from Tonmeister Andrew Lipinsky. Back in the 1990's I modified a Beyer 740 condenser mic for him. I used quality jfets and caps, but left the BC bipolar transistors in place. Andrew said the mic opened up but now had some 'fizzies' in the top end he didn't like.

He sent it back and after a PITA rework, I replaced the bipolar BC5xx transistors (mostly Philips) with Hitachi and Toshiba low noise discretes. The hiss was slightly reduced but more importantly, the 'fizzies' all went away. That mic sounds beautiful now. BC parts are replaced here on Studer consoles and tape machines as well, it does help.

BTW, that Ramsa uses a bipolar power supply, + - 15 volts.
Old 5th July 2015
  #7
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I cleaned my monitor. ok it is a dual rail power supply.
that means the transistors have to be a matched quad. this circuit should have been set up with a matched transistor array ic instead of discrete transistors.
I can also see where you think the bc transistors have a lot of 3rd harmonic noise, but it only is like that when someone uses them in circuits with high impedance collector loads (like above 100k). this one is like 7k on the collector so it shouldn't be too bad and 4560's don't like below 2k on the input so there wouldn't be much more than some harmonic differences if any between op amps. the drift on the emitter and collector resistors would be a concern as well as the transistors drifting from them being matched too.
other things that I think is slightly unusual. why the switch intersection between c12 and c13 in the first place? and the loading section there seems to be kinda redundant to me. I would think there would be some drawbacks having CRC network in the line stage.
Old 5th July 2015
  #8
It's a 1970's instrumentation amp mic preamp, sort of like the Brits used to do before they all discovered Paul Buff's work. Matched devices would improve CMRR and might allow the direct coupling of the gain set pot, but other problems like high THD and noise will still overwelm that.

It's best left alone and better candidates found for real world improvements.
Old 6th July 2015
  #9
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yea it would be better to use a step up transformer like a 200:47K into ic2. ditch ic1 and the transistors and rework the input level control into the negative feedback circuit of ic2with the op amp upgrade the offset voltage would be low enough to ditch the output cap ( c12) you might have to adjust r21 value if the output has dc offset voltage.

but just like Jim said, there are better mic preamps to mess around with this way....
Old 9th July 2015
  #10
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ulysses's Avatar
Quote:
Originally Posted by drtechno View Post
why the switch intersection between c12 and c13 in the first place? and the loading section there seems to be kinda redundant to me. I would think there would be some drawbacks having CRC network in the line stage.
The switch is there so the path doesn't pass audio while the gain control is switching (This switch section goes open at the same time the other sections do). It intends to reduce pops while switching the gain control. And the loading resistors on both C12 and C13 are there for the same purpose - by keeping those caps grounded, they hoped to reduce popping as those caps get disconnected and reconnected. I haven't encountered this product myself so I can only guess how effective each of these methods was toward reducing switch noise. I would suppose they had a 4-pole switch where a 3-pole would have sufficed, so it was worth a shot to see if this switching scheme improved the popping slightly.

Weary, do you notice a significant pop when you turn the gain switch?
Old 9th July 2015
  #11
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Hi
The scheme as proposed by Ulysses would be really cunning IF the switch wafer that does this last section is Break Before Make while the others are Make Before Break which would mean the signal gets disconnected before the other sections actually change.
Matt S
Old 28th May 2020
  #12
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I am interested in modding my Panasonic WR-500 mixer to have direct outs for each channel. I can't find any information on the service manual or schematic other than what was posted here. Does anyone have the full service manual pdf available?
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