PRR-176 REV 4 Build Thread ***Manual up!***1/16/14 - (chk 1st post)

[mrdarwin]

Anyone using the Don Audio power transformer?  I assume the 18VAC 1.2A secondary is used for the tube heaters.  Is the 18VAC 0.4A secondary adequate for the op-amps?

I use don audio xformer and it's ok for me... for the while

 

[abechap024]

leave 22p, same results...

output pots don't work with DOAS but work with ne5534

The DOAs might need more bypass capacitance to be stable. C89 and C42. Try 100pf or more.

 

[abechap024]

abe,

this enough poweer for the build? http://www.antekinc.com/as-0518-50va-18v-transformer/

That will give about 1.4 amps per side of 18v. That should be ok.

 

[mrdarwin]

leave 22p, same results...

output pots don't work with DOAS but work with ne5534

The DOAs might need more bypass capacitance to be stable. C89 and C42. Try 100pf or more.

I will try with 120pF, they're in stock ...
but Ilfungo's unit works with doas (on output?)

it's strange, output pot works with ne5534, but not with doa... in max position: distortion but never gain...
I'll tell you!
a solution for bypass?

 

[ilfungo]

Hi ABE!!!
Can you help us with bypass level problem?
Thanks

"I will try with 120pF, they're in stock ...
but Ilfungo's unit works with doas (on output?)"

Mine is working but I'm using not ABE DOA but PPA doa ( Pier Paolo Abbate italian forum member)...

ABE helps us!!! :

 

[e.oelberg]

So here is the state of my unit... It can pass audio without too much damage. I used stepped logarithmic 10 K pots. Only minimal gain changes on the first 20,steps and thwn distortion on max level. Sidechain works sort of. While the VU meters just dance a little there seems to be a little gain reduction.... But not like it should...output stepped log pot also doesn't work. It covers a relative small gain range. Log pot seem wrong. Wasted quite a lot on that... Guess it needs linear. Yes and the power regulators are very hot. Around 80 degrees i"d say. Despite the heatsinks. Nevertheless the box i built looks great !

Cheers, nicholas

 

[ilfungo]

Hi e.oelberg
Also I've made a 10k stepped for in/out...
I've built 4 PRR176 ( 2 x REV 1and  2 x REV 2) but I don't know why this rev have something strange....
My unit is working but there is something in the sound that I don't understand...

 

[e.oelberg]

whats mean working, are the log pots usable ? whats about compression? Whats about VU ?

I think something is wrong with my sidechain

 

[ilfungo]

I mean that the compression is as he should
I mean that the compression is as he should
Can I have 10db of compression without problem..

 

[e.oelberg]

okay, maybe there is something wrong with the diodes

 

[abechap024]

I don't know why the gain would go down when you put the unit in bypass. Mine doesn't do this. Check the CV when you switch it into bypass, what opamps are you using for sidechain?

Yes you need Linear on output.

Boost threshold to max to get the most gain reduction,

There will be NASTY distortion if you clip any of the opamp gain stages. This is natural, there is a lot of gain built into the unit. Hence the 470R input pad!

Look at the input stage schematic, make sure to change the 100k to 2k, I don't know what steps you used, with a regular 10k log pot the gain is very smooth, same with 10k output linear or rev log.

 

[abechap024]

Also, the meters should do more then just "dance".  With the VU meter you can peg it all the way and then some, it tends to bottom out around 25+db of gain reduction, but that is plenty for what this does...

I will take a look and do some more tests with my unit and post some videos. Like I've said before, the 470R input pad really helps the gain staging.

 

[e.oelberg]

thanks abe

I'm going to work that thru tomorrow:

I don't know why the gain would go down when you put the unit in bypass. Mine doesn't do this. Check the CV when you switch it into bypass, what opamps are you using for sidechain?

I don't have that problem as I have relay bypass

Yes you need Linear on output.

ok, sad, but I rip out the stepped log pots for linear pots

Look at the input stage schematic, make sure to change the 100k to 2k, I don't know what steps you used, with a regular 10k log pot the gain is very smooth, same with 10k output linear or rev log.

and I ckeck for that !

thank you !

 

[ilfungo]

Thanks Abe
I'm using TL072 for sidechain.
What is CV?  :-\

I've checked my stepped pot and for some error I've built 2 REV log and seems to work well.

Thanks

 

[mrdarwin]

Same problem with my pots, 10klog stepped too.
Tried with 10klinear, not better...

Ilfungo, how do you build a rev log stepped pot? Have you a calculator? thanks

 

[e.oelberg]

curve of my stepped pots I bought on ebay

http://www.ebay.com/itm/DACT-Type-21-Stepped-Attenuator-Potentiometer-10K-D-shape-shaft/191051183979?_trksid=p2047675.c100011.m1850&_trkparms=aid%3D222006%26algo%3DSIC.FITP%26ao%3D1%26asc%3D19852%26meid%3D4918362910359257820%26pid%3D100011%26prg%3D8938%26rk%3D0%26rkt%3D10%26sd%3D201010290851

these pots are hong kong built 21 step stereo. So I'll try to swap the pcbs for rev log

 

[ilfungo]

I've build them with Lorlin (shorting tipe) :
http://www.quadesl.com/attenuator.html
but I've inverted the steps:
1-2 for me are 11-12

on the first positions I've the big changes...
I don't know if is a true REV LOG but for me it's work..

On dual mono mode the channel are well matched
Stereo Mode
I don't know why the sound is more aggressive ( hard compression) and the channel go out matching.
I've seen that also the threshold trimmer are linked
Can be this the problem?
Thanks

 

[mrdarwin]

Here I am!
pics of unit:

so:
1) bypass: the switch spdt disconnect the sidechain and connect it to the ground(schematic). Simply disconnect sidechain and not connect to ground seems work like a bypass for me! imho, bypass relays are a better choice if you want a true bypass.

2) DOAs: I tried with 120pF, but it really seems that output doesn't work with abe's DOA(???) I will try with another DOA like Ilfungo when I 'll have time to build.

3) stereo link increases compression a lot! We can match unit with threshold trim, but the unit seems more stereo matched "without" stereo link. Ilfungo, do you use 470R pads? I don't use for the moment

4) hum on right channel... grrr

5) output symmetry pots don't work (in my unit... and you guys????)

6) thank you Ilfungo for the link. I use stepped lorlin mbb attenuator with custom pcbs (see pics) with the excel calculator of silen:art

7) tried log or lin 10k pots on output the result is really the same... big changes only on last positions... with a rev log big changes only on first postitons?

your turn!!!

 

[e.oelberg]

mrdarwin

your mains go underneath the pcb to your mains switch in the middle of your frontpanel.
I guess thats how you get your hum.

btw, this is one of the few things which my unit doesn't do and I like it that it doesn't do that.
There are also other things it doesn't do, which I hope to make it do soon.

 

[mrdarwin]

mrdarwin

your mains go underneath the pcb to your mains switch in the middle of your frontpanel.
I guess thats how you get your hum.

btw, this is one of the few things which my unit doesn't do and I like it that it doesn't do that.
There are also other things it doesn't do, which I hope to make it do soon.

You re probably right, will try to move...
hum only on channel two, less hum when I change tube... I dont know if power the tube in serial is a good thing...