Gain control in V672

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moqtev

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Hi all!

I'm racking some Telefunken V672 as preamps. I would very much like some thoughts on the gaincontrol - would you use a pot or a stepped switch?
In my test setup I'm using a 250K log pot - but there is some noise in the lowest position.
I don't know why but somehow I think that a more elegant solution would be:
20db switchable pad and a stepped switch for gain (with 6-8 steps - what R values would be good?) and then an output volume pot (stereopot but what value??).

What are your thoughts and experience? :?

Morten
 

rbaker

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Apr 27, 2005
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I had my v672's racked up by oliver and he used a pot.

I don't have much else to offer, I'm sorry. They sound GREAT though.
 

Tillmann

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Lotta info on this already. Did you do a search?

From what I've heard, it would be worth your time to try a 470K (or thereabouts) pot on the input before buying a stepped switch, unless you have your heart set on the swankness of a stepped switch.

T
 

moqtev

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Yes I've searched and read a lot of threads an the V672 here but it would be nice to hear from someone who have completed a project and worked with it because there seems to be a lot of different ideas on this :)

I will try your 470K- thanks

Morten
 

matta

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Just put a JLM Audio Go Between in front of the V672, gives you +48V, 20dB PAD and Phase. I then used a 470K pot for Gain Control (RG on the Schemtic) as Tillman said, had them running like this for a few months and all is well. Simple, elegant and sound great.

Cheers

Matt
 

Greg

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marthie_08:

I'm not quite sure I understand your gain control scheme. If you have a 500K pot in parallel with a 500R resistor... the 500R is so much lower than the 500K pot that it will dominate the combination and basically fixing the parallel at 500R. Thus fixing the gain. I suspected this and verified it when racking a V672 tonight. Am I missing something?

Thanks.
 

mattmoogus

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We just racked a pair of these with some Telefunken EQs and the normal U pad on the Go Between doesnt work because of the strange way the input circuit of the 672 works. I think we ended up having to insert a 2k resistor on each leg (with NO shunt resistor) to get the -26db pad we needed for unity gain thru.
Joe will be posting a thread with all the details on this and how the gain works on the JLM forum soon, as there seems to be a lot of mis-information floating around about these modules.
They do sound great though when theyre workin properly!

[email protected]
 

JLM Audio

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Greg, the pot and the resistor should have the same value, if I remember correctly, I must have made a mistake in the drawing, will check it out... I guess they are both 500k not sure though... I think I got the idea from some forum, to change the law of the pot, check this:
http://sound.westhost.com/pots.htm#chg-law
Adding a resistor to a linear pot will not work properly in an application that only uses a pot as a variable resistor like the V672. The V672 only needs a normal 500k log pot for its gain control as it feeds negative feedback level into the input transformer to reduce gain. It should be wired as a variable resistor so it is 0ohm when at minimum gain. The 500k pot will give you 36dB to 66dB gain control. When using a Go between it is best to change the 470R resistors on each side of the pad switch to 2k to make a 26dB pad. None of the resistors are needed between pins 1 to 4 on the V672 as the input impedance even without them is still above 600ohms. Just wire them as shown below. We also removed the original gain trim pot completely and strapped the wiper tag to the negative end (pin 4 to pin5 on circuit) so we could get all the negative feedback to the input transformer to get the 36dB minimum gain for more gain range.

JLMV672connections.jpg


Photo of a rack we just did with 2 x V672 and 2 x W695 in 1 rack unit high case. Also has 2 x 1Meg Active DI inputs and internal 100v to 240v mains power supply.
Telefunken%20rack%20640.jpg
 

AndyP

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I tried putting 40ohm resistors between pins 1 & 3 and 2 & 4 using the unmodified JLM go-between (with 470 ohm resistors) and the pad seems to work fine although I'm not sure how much attenuation I'm getting (enough for my needs). This is on an older version of the V672.

BTW, does this kind of pads change the load a microphone "sees" and, consequently, its frequency response?

Andy
 

Greg

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After finishing mine last night, I did not like the range and taper with the 500K linear pot/500K shunt resistor combo. Due to the linear taper, the second half through the gain sweep make little change in gain. I dug around my parts for a 500K log but didn't have one. :sad: And I think my max gain was around 47dB or so. I'll have to get a parts order going.

Joe, so is all this talk about "zero ohm input impedence" incorrect? From the schematic I got from kubi's site, it's looks like the input transformer is 1:2, so there has to be some impedence being reflected from the secondary. Can you comment on this a little?

I'm referring to the V672/1 document on this page:
http://audio.kubarth.com/rundfunk/index.cgi

There seems to be alot of conflicting info out there on the V672 so a little discussion on them would be very helpful to me.
 

JLM Audio

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Joe, so is all this talk about "zero ohm input impedence" incorrect? From the schematic I got from kubi's site, it's looks like the input transformer is 1:2, so there has to be some impedence being reflected from the secondary. Can you comment on this a little?
Hi Greg sorry for the delay in responding but it is beyond busy here at the moment.

Kubi is correct in the zero ohm topology for the V672. But the way the V672 does this has several loses due to the transformer winding resistances and coupling and overall gain of the first gain stage and the setting of the 500k gain pot. So we did lots of computer and listening testing without any resistors and got great results even with some mics that have a hard time usually driving 600ohm loads and the V672 sounded and tested great.

]
Telefunken%20rack%20640.jpg


The owner of the rack shown in the photo above is in the USA and had this to say about them in a email
"Damn dude.......Damn. This thing freaking rules!! Super clean install, as always. The 672's sound fat and have plenty of gain, and the eq's are so damn cool. Good call on separate ins & outs. I love this thing. Anyway thanks for everything. You'll be hearing from me soon. I have closet full of RCA stuff I'd love to start using again, "BA" series stuff (21,31,43,41,etc). They're screaming to get JLM'd.

I tried putting 40ohm resistors between pins 1 & 3 and 2 & 4 using the unmodified JLM go-between (with 470 ohm resistors) and the pad seems to work fine although I'm not sure how much attenuation I'm getting (enough for my needs). This is on an older version of the V672.
The GO between pad unmodified will give about 11dB PAD with the V672 and no resistors on pin 1&3 and 2&4. Changing the 2 x 470R to 2k mentioned up the page will give you a 26dB pad.

BTW, does this kind of pads change the load a microphone "sees" and, consequently, its frequency response?
Yes but there is not much you can do about that with the V672 other than switch a load resistor in when the pad is on but even that will sound different.
 

Greg

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Thanks kubi and Joe.

I'm still waiting to get the V672s I racked backed so I can make the changes. For the sake of time, I didn't do any frequency response plots. But after I get them back and make the changes, I'll definitely plot the spectrum using RMAA...
 

q2audio

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Aug 20, 2005
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USA
In my experience, the more gain you set these up for the more the 20Khz response droops. Too much series resistance caused the same effect.

I like a 20dB pad followed by 82 ohm series resistors just before the input. I also found that a 250K LIN pot works fine for a gain control - even though laudio/og would seem more obvious. Much over 50-ish dB of gain and the top end droops quite a bit...

About the pad: it's been a while since I did this, but IIRC, the normal 20dB pad did give very close to 20dB of attenuation - but it must be measured at the output not the input. There was some loading effect on my generator. I have a different set-up now and should re-run these tests.

Tim
 

barthman.de

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May 26, 2007
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Erfurt / Germany
I'm racking 4x V672 too. I use Grayhill 12 step rotary switches with 3 decks to realize 35 db and 65 db mode. Here the first photos:

v672_3.jpg


v672_4.jpg


rs.jpg


I hope I'll finish the wiring till Dezember ;)
 

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