the Poor Man 660 support thread

You should really ask, what is similar between a PM670 and Fairchild 670. And the answer is: the general topology to achieve Vari-mu compression, and nothing else.

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With pimp "Scamp" board is possible to have same time constant values and that's one step closer to Fairchild 670 

Kingston said:

I have found it extremely odd that so many people here have labeled their poormans with the Fairchild logo. It's like you build a really nice custom chopper from the ground up and stick a Rolls Royce logo on the gas tank.

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That's exactly the reason.. Probably like the others who already stamped the FC logo, I thought it differs mostly only by the side chain and use of up-to-date parts + cheaper tubes (and also less, I thought there are more tubes in parallel use in the original or something like that). Hearing this there won't be a Fairchild logo when she's ready one day (with the scamp too)

I was working with my poorman in the studio on a masteringjob and i noticed it has a low end roll off.
Not like the low end is gone or something but it sounds a bit like the roll of is maybe about 3db at 50hz or something like that. (didn't measure it yet, i can't get rmaa to work right now)
For the rest the unit sounds great and smooth but i just noticed that roll off.
Anyone else have that problem? maybe change some cap values?  ???

dagoose said:

Anyone else have that problem? maybe change some cap values?  ???

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http://www.groupdiy.com/index.php?topic=28274.msg414809#msg414809

Thank you for choosing the Volker search engine  ;D

[silent:arts] said:

dagoose said:

Anyone else have that problem? maybe change some cap values?  ???

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http://www.groupdiy.com/index.php?topic=28274.msg414809#msg414809

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No, that is not it.. it's exactly the same on both channels, otherwise i would have made the same soldering error on both channels exactly the same.  ;D
It's not like really much but you hear something is going on/missing in the low end, i will post an example later on.

dagoose said:

the same soldering error on both channels exactly the same.  ;D

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These things DO happen sometimes .....

No but seriously, you have a scoop right? Send a square wave in the poorman and see where the low cut occurs... Should not be too hard to find out if you ask me...

It's not like really much but you hear something is going on/missing in the low end, i will post an example later on.

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If the roll off is subtle then measure the "beast" first ( yes square will tell you how good the situation is), but I suspect that bass roll off can be caused by big tubes  mismatch. Edcor has no much space to compensate DC current difference. As far I know tx is not gaped and total mismatch shouldn't be more than few ma's. Just my 0.2c

dagoose said:

I was working with my poorman in the studio on a masteringjob and i noticed it has a low end roll off.
Not like the low end is gone or something but it sounds a bit like the roll of is maybe about 3db at 50hz or something like that. (didn't measure it yet, i can't get rmaa to work right now)
For the rest the unit sounds great and smooth but i just noticed that roll off.
Anyone else have that problem? maybe change some cap values?  ???

Click to expand...

There's no cap in the signal path so the problem cannot be this.
During my tests I didn't notice any bass roll-off but the load on the PM670 output was more than 10K so :
Where is placed the PM670 in your mastering chain ?
What is its load on the output ?

My mistake... i thought i heard a roll off but it was more like a 'low freq roll up' on another compressor i have.  ???
So... actually it is flat, when i bypass (hard relay) i still have the same response as when enganged so nothing is wrong. (ok.. except for the vu meter null adjust which is still a bit to hard to get zero but that is no problem.  ;D )
But anyway... thanks!  8)

But anyway... thanks!

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Great news 

Very strange: My VU-Meter (Sifam AL22) is pegging hard to the right even when NOT CONNECTED  ???
After I couldn´t get it trimmed to zero (taking out R6 and replacing it with various trimmers) I took out the rectifier diodes and resistors. I then could get the needle to zero but tracking was very poor so I wanted to try the VU route once more.
I have a few more AL22 but they all behave the same: when I come near the meter connector with the leads the needle starts pegging to the right (I guess it is some interaction with the 5687), I just don´t understand it...
OR is it just better to get the recommended VU-Meter (Sifam AL20), but why should this VU-Meter react different then?
Thanks a lot, Emre

veermaster said:

Very strange: My VU-Meter (Sifam AL22) is pegging hard to the right even when NOT CONNECTED  ???
After I couldn´t get it trimmed to zero (taking out R6 and replacing it with various trimmers) I took out the rectifier diodes and resistors. I then could get the needle to zero but tracking was very poor so I wanted to try the VU route once more.
I have a few more AL22 but they all behave the same: when I come near the meter connector with the leads the needle starts pegging to the right (I guess it is some interaction with the 5687), I just don´t understand it...
OR is it just better to get the recommended VU-Meter (Sifam AL20), but why should this VU-Meter react different then?
Thanks a lot, Emre

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You've got to understand how's the VUmeter is tracking gain reduction (maybe you already understood that but some of us can be interrested) :
A vu meter is a AC voltmeter (with the rectifier build in). Through R6, the current pumped by the 6BC8 creates a voltage you measure. As the tubes are pumping less current when compressing the voltage drops and the needle goes left. Some little maths : If we call Ib the DC current pumped by the tubes and Ia the current pumped by the tubes to create audio signals voltage on R6 can be calculated with this : Ur6 = [Ib + (Ia - Ia)] x R6.

You can use nearly any Vu or voltmeter here to track compression. On mine, I left R6 on the board and connected on parallel a 500 ohm pot (with 2 points wired together to react as a single resistor). One point of the pot is connected directly to the vumeter, the other point feed the Vu through a 3K6 or 3k9 ohm resistor. It works ! And my Vumeter is an old PEKLY (french brand unfortunately disapeared a long time ago)...



Try this if you didn't and tell us what's happening  .

PS: the needle reacts weirdly only when the 6BC8 are overdriving audio (I mean with no compression).

Thanks for your reply lolo.
I will try your method as soon as I´m home but what really puzzles me is the fact that my vu-meter pegs hard right even though it is NOT connected. As soon as I come near the 5687 tubes with the leads of my meter the needle starts to move like there is some sort of induction going on (excuse my probably nonsense talking here). How is that possible?

I'm not an expert in this but maybe there's some electrostatic field generated by the tubes... I'm surprised by the fact it can interract with the meter  :... Think about some metal shield there if the problem persists. But before, test without any shield  . This will certainly learn us something  ;D !
Your problem is really weird  .

Well maybe a little about theory may help.. I wanted to post earlier when Lolo mentioned hum but I was short in time.
Electromagnetic fields decay with the square of distance, so heater wiring runs should be as far away as possible from signal circuitry as possible, and only come up to the valve at the last possible moment and in the most direct manner possible.
Valve sockets should be oriented so that the pins receiving heater wiring are as close to the chassis wall as possible, and heater wire should never loop round a valve (except for rectifier valves, where hum is not an issue).
Electrostatic fields and heater wiring....
The electrostatic field is due to the voltage on the wiring.
Heater wiring should be pushed firmly into the corners of the (conductive) chassis, since the electrostatic mirror at the corner tends to null some of the electrostatic field. Heater wiring must not run exposed from one valve to the next, but should return to the corner of the chassis to re-emerge at the next valve.
Hope this helps  ;D

Hi,
I'm back in front line with this monster !!

here is something I have found that it's not clear.
I have had a look at the data sheets for the MPS42 and MPS92 and the maximum VCBO and VCEO are 300 while on the base of Q1 there are effectively 370VDC.
would this 70VDC differece cause the transistor to blow?

can anyone,very kindly, take some static measurements around Q1, Q2, Q3, Q4 e Q5, please?

Many thanks for the help

Regards

AW_music said:

Hi,
I'm back in front line with this monster !!

here is something I have found that it's not clear.
I have had a look at the data sheets for the MPS42 and MPS92 and the maximum VCBO and VCEO are 300 while on the base of Q1 there are effectively 370VDC.
would this 70VDC differece cause the transistor to blow?

can anyone,very kindly, take some static measurements around Q1, Q2, Q3, Q4 e Q5, please?

Many thanks for the help

Regards

Click to expand...

not exactly.. on the base of Q1 is around 300v and on the collector around 250V. So there is 50V...

Excuse me Moby,
I'm talkin about MPS92....... =Q1
is impossible to have 300V there !!!!
SO form trafo i have 270Volt AC in .
Then after D1-2-3-4 i will find near 370 and then at base of Q1 i will have near 370.

Whats wrong ???

Hey Moby would you be so kind to take some REAL static measurements so maybe i can figure out whats going on here ?  ???
Any other good samaritan !!! : :

Thanks for help

Regards

About my meter problem:
I now use a 15V meter shunted with a 5K trimmer (took out the 22K shunt).
Works fine now. Thanks for all your help!
emre