RCA OP6 Clone

Looks like someone's reverse engineered the unobtanium attenuator in the OP6 and made a clone:

http://www.gearslutz.com/board/product-alerts-older-than-2-months/738804-ssa-op-6-1-worlds-first-i-think-rca-op-6-clone.html

Care to enlighten further if there is some big deal to be found there? "unobtanium gain attenuator" reads to me like some strange log law discontinued pot or a stepped rotary. Looks like so many other DIY version of some classic. cinemag in, edcor out, three stages of gain, tube rectifier PSU. A common equation really, or maybe I've read too many emrr posts. Probably sounds like any great tube pre should.

I am interested in what an "unobtanium gain attenuator" would be.

abechap024 said:

I am interested in what an "unobtanium gain attenuator" would be.

Click to expand...

Looking at the schematic it seems to be a rather neat switched combined feedback and attenuator control. In the 0 position the first stage output is shorted to ground so there I no output. At the 1 position the first stage gain is set by the 1.2meg feedback resistor and the 47k resistor in series with the transformer secondary at about 28dB. The rest of the switch then looks like a several stage PI attenuator. It is not possible to tell how much this attenuates because n components values are given but we can make a guess. First we look t the output stage and note its gain is about 26dB. If the output transformer is a 15k/600 type this has a loss of about 14dB giving an overall output stage gain of about 12dB. If the input transformer his a gain of 20dB then the total gain available is 20+28+12=60dB. The question now s w much of this do we want to lose in the attenuator? Since this is the lowest gain setting of a volume control good guess might be 40dB. With a 20 step switch we could then proceed in 2dB steps up to zero attenuation. However this does not take account of the extra gain we get t the max volume position. At this position there s no attenuation of he first stage output but the full 40dB attenuation is in the NFB loop which in theory increases the first stage gain by 40dB. The gives control a 80dB range  which is twice what we want.so let's change the maximum attenuation to 20dB. So, at the 1 position we have 28dB gain an 20dB loss for an overall 8dB gain. At the 20 position we have no loss and 48dB gain for an overall 48dB gain giving s the desired 40dB range.

This is probably close to the actual situation. My only reservation is that 48dB from the first stage is probably close to or equal to is open loop gain.

Cheers

Ian

letterbeacon said:

Looks like someone's reverse engineered the unobtanium attenuator in the OP6 and made a clone:

http://www.gearslutz.com/board/product-alerts-older-than-2-months/738804-ssa-op-6-1-worlds-first-i-think-rca-op-6-clone.html

Click to expand...

Reading the article in detail it appears he did not. Instead he rejigged the first stage to be a clone of the BA-2 and just followed it with a 100k pot. At the end he calls it a BA-2/OP6 hybrid.

Cheers

Ian

ruffrecords said:

letterbeacon said:

Looks like someone's reverse engineered the unobtanium attenuator in the OP6 and made a clone:

http://www.gearslutz.com/board/product-alerts-older-than-2-months/738804-ssa-op-6-1-worlds-first-i-think-rca-op-6-clone.html

Click to expand...

Reading the article in detail it appears he did not. Instead he rejigged the first stage to be a clone of the BA-2 and just followed it with a 100k pot. At the end he calls it a BA-2/OP6 hybrid.

Cheers

Ian

Click to expand...

I think theres two different ones involved there, the preservationsound.com build which is what you're talking about above and the one by the thread starter ben-hur which has a panel meter and several switches.

Yes as MattisD says, the BA-2/ OP6 hybrid was an earlier attempt at getting close to the OP6 sound.  Here's the page on that build: http://www.preservationsound.com/?p=2576

Kingston said:

Care to enlighten further if there is some big deal to be found there?

Click to expand...

Oh I dunno if it really is a big deal.  I remember reading a while back that the OP6 is 'impossible to clone' because no one knows what the attenuator after the first stage was.

Looking at the schematic it seems to be a rather neat switched combined feedback and attenuator control. In the 0 position the first stage output is shorted to ground so there I no output. At the 1 position the first stage gain is set by the 1.2meg feedback resistor and the 47k resistor in series with the transformer secondary at about 28dB. The rest of the switch then looks like a several stage PI attenuator. It is not possible to tell how much this attenuates because n components values are given but we can make a guess. First we look t the output stage and note its gain is about 26dB. If the output transformer is a 15k/600 type this has a loss of about 14dB giving an overall output stage gain of about 12dB. If the input transformer his a gain of 20dB then the total gain available is 20+28+12=60dB. The question now s w much of this do we want to lose in the attenuator? Since this is the lowest gain setting of a volume control good guess might be 40dB. With a 20 step switch we could then proceed in 2dB steps up to zero attenuation. However this does not take account of the extra gain we get t the max volume position. At this position there s no attenuation of he first stage output but the full 40dB attenuation is in the NFB loop which in theory increases the first stage gain by 40dB. The gives control a 80dB range  which is twice what we want.so let's change the maximum attenuation to 20dB. So, at the 1 position we have 28dB gain an 20dB loss for an overall 8dB gain. At the 20 position we have no loss and 48dB gain for an overall 48dB gain giving s the desired 40dB range.

This is probably close to the actual situation. My only reservation is that 48dB from the first stage is probably close to or equal to is open loop gain.

Click to expand...

Very interesting read, thanks Ian!

MatthisD said:

ruffrecords said:

letterbeacon said:

Looks like someone's reverse engineered the unobtanium attenuator in the OP6 and made a clone:

http://www.gearslutz.com/board/product-alerts-older-than-2-months/738804-ssa-op-6-1-worlds-first-i-think-rca-op-6-clone.html

Click to expand...

Reading the article in detail it appears he did not. Instead he rejigged the first stage to be a clone of the BA-2 and just followed it with a 100k pot. At the end he calls it a BA-2/OP6 hybrid.

Cheers

Ian

Click to expand...

I think theres two different ones involved there, the preservationsound.com build which is what you're talking about above and the one by the thread starter ben-hur which has a panel meter and several switches.

Click to expand...

Yes you are right. I got confused I think because both posts are on gearslutz. However, I cannot seem to find a tilt eq on the original.

Cheers

Ian

You can do crude version of the unobtainium using a 1meg pot with a 110k reis to to ground at each end. One and is the output, he other connects to the feedback and the wiper goes to the first stage output. With the wiper at the feedback end you get the minimum gain and about 20dB attenuation. With the wiper at the output end you get no attenuation and 20dB extra gain. Loading effects on tbe first stage have been ignored.

Ian

Cheers

MatthisD said:

The variable pot/attenuator looks to be shown in a number of positions which gives the impression that it is a switch with resistor network/ladder but it is actually continuously variable, the resistor to ground part of the pot must be whats 'unobtainium' about it. Its mentioned in the first comment at the bottom of the preservationsound page. Maybe there is a switch into the 0 position at the CCW side for 'off'.

Click to expand...

So are you saying you have actually seen one and the cvontrol is really continouous and not switched?

Also mentioned in the build there is that there is 95dB total gain with 5:1 output. I don't know but the output transformer ratio could be 10:1 in the OP-6 which would lower total gain by a few dB.

Click to expand...

I can just about believe that. This appears to have a 250 ohm input which might be used with the typical 50 ohm mics of the day so more gain would be needed. Input transformers had ratios up to 1:30 for 50 ohm inputs which gives 30dB gain. A 5:1 at the output implies a 14dB loss and I think my calculation of the output stage gain was a little conservative so including the transformer I think it is nearer 14dB. So, taking 30 + 14 from 95 leaves 51dB to be had from the first stage which is 354 times. The 1620 gm is such that with a 220K plate load its gain is unlikely to reach 300 times. So I could believe 90dB but unless the output transformer is less than 5:1 I find it hard to credit 95dB.

Cheers

Ian

MatthisD said:

ruffrecords said:

MatthisD said:

The variable pot/attenuator looks to be shown in a number of positions which gives the impression that it is a switch with resistor network/ladder but it is actually continuously variable, the resistor to ground part of the pot must be whats 'unobtainium' about it. Its mentioned in the first comment at the bottom of the preservationsound page. Maybe there is a switch into the 0 position at the CCW side for 'off'.

Click to expand...

So are you saying you have actually seen one and the cvontrol is really continouous and not switched?

Click to expand...

No not in the flesh, but if you count the video here http://worldclassaudio.com/ as authentic then yes.

This was the comment by Kevin Ink
"Firstly the attenuator is made up of hand wound wire resistors built into something similar to a Daven attenuator"

Click to expand...

I have looked at that video several times now and I am convinced it is a switch. A very nice smooth switch but a switch nonetheless. I can clearly see places in the early part of the video where the knob clicks into place. If this is a Daven attenuator then it will be very smooth acting and could easily be mistaken for a pot.

Cheers

Ian

MatthisD said:

external shot of attenuator here:
http://i109.photobucket.com/albums/n44/F-64/DSC_0031.jpg

I thought the builder meant 95dB measured, but now I see he said 95dB 'apparently'. Initially I thought the OP-6 had 90dB max gain which is why I thought it could be a 10:1 output not a 5:1 which was used in that clone. You might be right with the ratio being 5:1 or 4:1 but I can't find info on the actual output transformer ratio. Its mentioned here again by emrr;
http://www.gearslutz.com/board/high-end/491665-rca-op-6-powerhouse-4.html

Also the Hammond choke DCR is nearly 4K.

Click to expand...

That attenuator looks like a Daven switch type to me.

Incidentally on the video they say the OP6 has up to 90dB gain so I think that is the more likely figure.

Cheers

Ian

letterbeacon said:

Looks like someone's reverse engineered the unobtanium attenuator in the OP6 and made a clone:

http://www.gearslutz.com/board/product-alerts-older-than-2-months/738804-ssa-op-6-1-worlds-first-i-think-rca-op-6-clone.html

Click to expand...

Apparently that's all it took to be official . . .

And a price "that's right" - call it $250 and that should do the trick. 

I think the real news for a reworking of a tricky clone of something worthy is here: 

http://www.groupdiy.com/index.php?topic=47703.0

MatthisD said:

ruffrecords said:

I have looked at that video several times now and I am convinced it is a switch. A very nice smooth switch but a switch nonetheless. I can clearly see places in the early part of the video where the knob clicks into place. If this is a Daven attenuator then it will be very smooth acting and could easily be mistaken for a pot.

Cheers

Ian

Click to expand...

That would mean alot of clicks, 5 or 6 between each dot? which would mean a lot of resistors.
Hopefully someone who has used one can confirm either way but it could be a clickless switch, a stepped pot whatever you want to call it. Make before break operation but with smooth travel. I watched it again but theres points where it moves smoothly over a very short arc.


These may be the type of hand-wound resistors that Kevin Ink referred to:

http://www.groupdiy.com/index.php?topic=41962.0

...21 contacts with, it looks like 20 resistors total, obviously configured as a dual pot in that unit. You can see on the OP-6 schematic that the lowest position is marked 20 and the top position marked 0 which is 21 total, theres 12 missing positions in between which is very confusing. The panel marking for the gain knob is marked 0-20 all the same. It seems to be making more sense to me now and that your instinct about it being a switch is right, something I didn't consider was that the contact wiper could move smoothly between positions and without clicks which is what I believe is happening, although you might not agree with that. I remember I made a Grayhill rotary switch work like this before by accident; reassembling it with the contact wiper upside down.

Click to expand...

I suspect it is a 40 or 41 position switch so there is one position between every numbered one. Some of these Daven switches have a very smooth action with a big wiper that probably does the make before break action. I saw a pic of one somewhere recently. If I can find it I'll post it.

Cheers

Ian

Here's a 51 way example:

http://www.ebay.com/itm/A-pair-of-Daven-6882-rotary-switch-51-position-detent-make-before-break-/271060473797?nma=true&si=pTk2OW%2FGUNS3AzDpbNELTpdORRQ%3D&orig_cvip=true&rt=nc&_trksid=p2047675.l2557

Scroll down and look through the pics

Cheers

Ian

Also the Hammond choke DCR is nearly 4K.

Click to expand...

Any more detail on the choke for building something like this?

http://www.hammondmfg.com/153.htm

dmp said:

Also the Hammond choke DCR is nearly 4K.

Click to expand...

Any more detail on the choke for building something like this?

http://www.hammondmfg.com/153.htm

Click to expand...

just pick something above your max current needs and the DCR to suit the voltage drop you want. remember to check size, some of them can be quite large and heavy.

using a B+ chokes is hands on. not critical at all.

[edit]

use these to compare what you get with a choke vs. a dumb resistor.

http://www.ampbooks.com/home/amplifier-calculators/RC-ripple-filter/
http://www.ampbooks.com/home/amplifier-calculators/LC-ripple-filter/

Please look at the schematic. This is an choke for the plate feed of the output tube.

Also, the reason for this based on what I've picker up from other people here - resistive coupling is the least efficient, while transformer or inductive (choke) coupling is much more efficient. This means the output headroom is much higher for a inductive or transformer coupled output stage. Obviously resistive coupled is the easiest due to the no-dc transformer 

dmp said:

Please look at the schematic. This is an choke for the plate feed of the output tube.

Click to expand...

Well those hammond chokes you linked are only designed for PSU filtering, so I thought you were asking about that. Don't expect full range audio from them. Chokes need to be specifically designed for that task, and those are no good for PSU filtering. And they are way more expensive, and harder to find. I only know of several lundahl plate chokes that are still made today.

Also, if you want the most optimal solution, use SE output stage. The choke and output transformer can be combined into one, and you still get the roughly doubled headroom.