ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #20 on: October 11, 2017, 06:09:22 PM »
Typically for a 34 dB gain range, the ideal taper should be 7%. Most commonly available RevLog are 15 or 20%. They make the CW part of the rotation quite clustered.
With 15% taper, you have 12dB on the left and 22 on the right.
You have to place a special order for the more desirable 10 or 5% types, unless you can buy spares from a manufacturer.

Mmmm, disappointing and, as we know, there's no point trying to slug it.

I suppose the next best thing would be a 12 way Lorlin in 3dB steps.

Cheers

Ian


bluebird

Re: Single tube mic pre with continuous gain control
« Reply #21 on: October 12, 2017, 02:35:57 AM »
I am toying with the idea of reducing the 47K feedback resistor to 22K or so. Cannot go too low else the output gets loaded and distortion rises but the noise might be a bit less at lower gains and I might be able to raise the second plate a little to extend the headroom.

You could also have a "high/low" gain switch. A small DPDT toggle could adjust the feedback resistor and output plate resistor. The UA 610 had an internal switch that had a similar function (just feedback resistor).
But I guess that defeats the whole purpose of having the large gain range in the first place.

A little off topic, there were some engineers working at the studio that had a bunch of UA 610 modules with various external power supplies. They had XLR barrel pads on the input of each one. They were -30db U pads. I told them they could switch the internal switch to the low gain setting and they looked at me like I was crazy.
Point being they were happy to trade more noise for the sound of more system gain.
I've noticed this preference with other tube gear.

scott2000

Re: Single tube mic pre with continuous gain control
« Reply #22 on: October 12, 2017, 03:14:44 AM »

A little off topic, there were some engineers working at the studio that had a bunch of UA 610 modules with various external power supplies. They had XLR barrel pads on the input of each one. They were -30db U pads. I told them they could switch the internal switch to the low gain setting and they looked at me like I was crazy.
Point being they were happy to trade more noise for the sound of more system gain.
I've noticed this preference with other tube gear.

I was reading here https://groupdiy.com/index.php?topic=7707.msg148522#msg148522  that too hot an input creates distortion in these 610s and a pad is recommended unless you want that fuzz effect? So is this just a lesser of two issues scenario or am I thinking wrong? Is there a difference in using the pad vs the internal switch that would make this a justifiable reason to pad the input or is just the sound of the gain  they're after I wonder.....

Just trying to wrap my head around things....Kinda following this thread......
« Last Edit: October 12, 2017, 03:26:04 AM by scott2000 »

ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #23 on: October 12, 2017, 03:25:06 AM »
You could also have a "high/low" gain switch. A small DPDT toggle could adjust the feedback resistor and output plate resistor. The UA 610 had an internal switch that had a similar function (just feedback resistor).
But I guess that defeats the whole purpose of having the large gain range in the first place.
Or do it at ac only with a parallel RC but as you say the gain range is already pretty wide and this will not add much.
Quote
A little off topic, there were some engineers working at the studio that had a bunch of UA 610 modules with various external power supplies. They had XLR barrel pads on the input of each one. They were -30db U pads. I told them they could switch the internal switch to the low gain setting and they looked at me like I was crazy.
Point being they were happy to trade more noise for the sound of more system gain.
I've noticed this preference with other tube gear.
I cannot find a 610 schematic right now but from memory the first stage has some NFB with a switch that selects two gain settings - not sure what  the gain values are. If the gain setting circuit only changes the NFB then there will be lower distortion at the lower gain setting so it definitely sounds like they are going for colour. But they may be fooilng themselves. Distortion in triodes is directly proportional to output level. If you drop the level by 30dB on the way in then the distortion will drop by 30dB. If they used the low gain setting without a 30dB pad and the low gain setting only drops the gain by 20dB for example, then the output stage will be driven 10dB harder and produce more distortion. Even so, the stage that produces the most distortion is the second stage because the level is greatest. Is the gain switch in the second stage or the first. Starting to ramble now. Need the schematic!!

Cheers

Ian
« Last Edit: October 12, 2017, 12:48:52 PM by ruffrecords »

rackmonkey

Re: Single tube mic pre with continuous gain control
« Reply #24 on: October 12, 2017, 11:46:03 AM »
FWIW, I've never seen an actual 610 production version schematic. The one John Hinson drew supposedly from Neil Young's console is floating around all over the place, but it's not the same as the more modern 610s I have. I finally got so frustrated at the lack of them that I pulled a PCB and reverse engineered it. I'm hesitant to post that given UA's "no schematics" policy, but I could tell you the differences between John H's and that one. Or PM it.

BT
Whether you think you can or you think you can't, you're probably right.

ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #25 on: October 12, 2017, 12:51:11 PM »
FWIW, I've never seen an actual 610 production version schematic. The one John Hinson drew supposedly from Neil Young's console is floating around all over the place, but it's not the same as the more modern 610s I have. I finally got so frustrated at the lack of them that I pulled a PCB and reverse engineered it. I'm hesitant to post that given UA's "no schematics" policy, but I could tell you the differences between John H's and that one. Or PM it.

BT

A current production 610 PCB is in the public domain, There is nothing wrong with you tracing its schematic from the PCB and publishing it but then IANAL. If you want to PM it to me that's fine.

Cheers

Ian

bluebird

Re: Single tube mic pre with continuous gain control
« Reply #26 on: October 12, 2017, 06:21:50 PM »
Is there a difference in using the pad vs the internal switch that would make this a justifiable reason to pad the input or is just the sound of the gain  they're after I wonder.....

Yes there is a big difference. Like Ian suggested the internal gain switch changes the amount of NFB in the first stage, output/plate of second triode to the cathode of first triode. A pad just turns down the input signal. It also changes the input impedance that the 12AX7 sees. The pad is supposed to mimic a mic with the first tube stage seeing a 150ohm resistor. The mic sees a couple build out resistors in addition to the 150ohm in a U configuration. Not everyone make pads the same way. So depending on what you load the first stage with, your sound can vary a lot with a pad. These engineers like the pad to have a lot lower of a resistor than a 150ohm. something like a 40 ohm. The UA 610 seemed to like seeing that more than a mic impedance. Gave a more "open" sound.

The input to the UA 610 is a UTC Ouncer 0-1 tapped at the highest ratio. I am familiar with the Hinson schematic and it actually also had a 47ohm slug in series with the primary. I'm not sure why they did this, I could guess, but the real modules did have this resistor.
I have the schematic around somewhere, I'll try to find it.

80hinhiding

Re: Single tube mic pre with continuous gain control
« Reply #27 on: October 12, 2017, 08:07:00 PM »
Thanks for sharing this Ian.

bluebird

Re: Single tube mic pre with continuous gain control
« Reply #28 on: October 13, 2017, 12:59:17 AM »
Here is a schematic I found floating around on the internet a while ago.

ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #29 on: October 13, 2017, 04:07:38 AM »
Yes. that is the one I found. Not sure just how accurate it is but the 12AX7 stage looks relatively conventional. The change of overall gain by changing a tap in the NFB network is a little unusual in that it does not change the headroom so it is unlikely to make any difference to the sound.

Cheers

Ian


scott2000

Re: Single tube mic pre with continuous gain control
« Reply #30 on: October 13, 2017, 10:36:53 AM »
Yes. that is the one I found. Not sure just how accurate it is but the 12AX7 stage looks relatively conventional. The change of overall gain by changing a tap in the NFB network is a little unusual in that it does not change the headroom so it is unlikely to make any difference to the sound.

Cheers

Ian


The thread I mentioned/linked earlier mentioned someone having an original 610 schem that was hand drawn??? Wonder if that's around somewhere...... It was CJ but, I'm not certain which CJ if there are more than the one I'm familiar with.....
« Last Edit: October 13, 2017, 11:34:54 AM by scott2000 »

mjrippe

Re: Single tube mic pre with continuous gain control
« Reply #31 on: October 13, 2017, 01:36:25 PM »
Here is a schematic I found floating around on the internet a while ago.

That is the Hinson schematic but someone cropped out his name, etc.  See attached.

bluebird

Re: Single tube mic pre with continuous gain control
« Reply #32 on: October 13, 2017, 02:10:06 PM »
At some point I traced a couple of old modules and the schematic is accurate. I think the only difference I saw was an 82K tagged on the top of the 250K pot. So its really about a 330K pot that only turns up two thirds of the way.

rackmonkey

Re: Single tube mic pre with continuous gain control
« Reply #33 on: October 13, 2017, 03:29:49 PM »
Ian, I PM'd you the schematic I traced out from one of my modules along with an annotated picture of the PCB. I also sent the John Hinson schematic linked above. I used that to compare against and it made the job easier.
Whether you think you can or you think you can't, you're probably right.

rackmonkey

Re: Single tube mic pre with continuous gain control
« Reply #34 on: October 13, 2017, 03:38:19 PM »
At some point I traced a couple of old modules and the schematic is accurate. I think the only difference I saw was an 82K tagged on the top of the 250K pot. So its really about a 330K pot that only turns up two thirds of the way.

There are actually a few other differences.  I just went back to compare the schematics so I could post them and realized that I listed actual resistor values on a separate sheet of paper. All I have on the schematic right now is the resistor number as labelled on the PCB. I didn't send you that list, Ian. I'll get that to you later today, then I'll compare values with the Hinson schematic and post the differences.

EDIT: I should say there there are a few more differences in the modern production modules. If you traced out a couple of older ones, Bluebird, they very well might be closer to what the Hinson schematic shows.
 
BT
« Last Edit: October 13, 2017, 04:17:07 PM by rackmonkey »
Whether you think you can or you think you can't, you're probably right.

ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #35 on: October 13, 2017, 03:44:48 PM »
At some point I traced a couple of old modules and the schematic is accurate. I think the only difference I saw was an 82K tagged on the top of the 250K pot. So its really about a 330K pot that only turns up two thirds of the way.

OK, that is a minor difference.

It is still a basic 12AX7 triode pair with overall NFB. The signal can be direct from the second stage or from a tap in the NFB network which will give a lower output - looks to be about 10dB lower is all. Either way, the second triode plate signal level is the same so the amount of distortion it produces is the same no matter what the gain setting.

With a 1:10 input transformer the high gain is about 47dB and the low gain about 37dB. Using a 30dB pad at the input will drop the gain to 17dB in the high gain setting so there is very little chance of getting significant distortion from the 12AX7stage. It is all going to be generated in the 12AY67stage.

Cheers

ian

bluebird

Re: Single tube mic pre with continuous gain control
« Reply #36 on: October 13, 2017, 06:21:46 PM »
Well that's the crazy thing, they are using the 50 ohm primary tap, so (50 ohm : 50K) is a 1:30 turns ratio, right?

And herein lies the subjective magic (good or bad). I've used the 600 tap and the pre behaved A LOT better in a lot of ways, Why they choose to use the 50ohm tap with a 47 ohm resistor had to be with a certain type of microphone in mind.

I'm guessing the new UA 610's use a Cinemag input and figured out it sounds more "open" with a more reasonable step up ratio, but I doubt as much as the circuit is the same, they will sound like the original modules.  I've said it before, the interfacing of the transformer and first gain stage is the bulk of the sound.
« Last Edit: October 13, 2017, 06:33:35 PM by bluebird »

ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #37 on: October 13, 2017, 06:36:22 PM »
Well that's the crazy thing, they are using the 50 ohm primary tap, so (50 ohm : 50K) is a 1:30 turns ratio, right?

And herein lies the subjective magic (good or bad). I've used the 600 tap and the pre behaved A LOT better in a lot of ways, Why they choose to use the 50ohm tap with a 47 ohm resistor had to be with a certain type of microphone in mind.

I'm guessing the new UA 610's use a Cinemag input and figured out it sounds more "open" with a more reasonable step up ratio, but I doubt as much as the circuit is the same, they will sound like the original modules.

Odd. If they used a barrel attenuator they usually look like either 600 ohms or 1K5 or so to the mic. Load one of these with 50 ohms and all you do is attenuate some more, maybe 10dB, but it makes no difference to the impedance seen by the mic. SO with a regular 30dB attenuator on the 50 ohm input you probably lose 40dB then gain 30dB through the transformer.

The only difference is the transformer is working on the 50 ohm tap so the frequency response will probably be compromised in some way. Maybe that gives the the warm sound they are looking for?

Cheers

Ian

bluebird

Re: Single tube mic pre with continuous gain control
« Reply #38 on: October 13, 2017, 06:55:16 PM »
The only difference is the transformer is working on the 50 ohm tap so the frequency response will probably be compromised in some way. Maybe that gives the the warm sound they are looking for?

Exactly, now imagine if you boosted the treble on that channel to brighten it up again. Here you could be adding more distortion or noise into the upper frequency range often described as "air".

If there is one thing I've learned, bandwidth above 20K (I would even argue 18K) has nothing to do with what people discribe as "openess" or "air".  Transformers, line amps micpres that quote specs out to 40K 60k whatever don't impress me. Maybe technically but I'm not expecting anything sound wise.
« Last Edit: October 13, 2017, 07:01:25 PM by bluebird »

ruffrecords

Re: Single tube mic pre with continuous gain control
« Reply #39 on: October 14, 2017, 02:20:21 AM »
Exactly, now imagine if you boosted the treble on that channel to brighten it up again. Here you could be adding more distortion or noise into the upper frequency range often described as "air".

If there is one thing I've learned, bandwidth above 20K (I would even argue 18K) has nothing to do with what people discribe as "openess" or "air".  Transformers, line amps micpres that quote specs out to 40K 60k whatever don't impress me. Maybe technically but I'm not expecting anything sound wise.

One thing that occurred to me is that the transformer is effectively unloaded and I know the frequency response of transformers varies with source and load impedance, I use Sowter, Jensen and Cinemag 1:10 input transformers on my mic pres and they all exhibit this. They are designed for a 150 ohm source and if you drive them from a 150 ohm source their response is ruler flat well past 20KHz. However, if you drive them from a lower impedance you can get a chunky peak in the 20 to 25KHz region - about 5dB or so which definitely 'enhances' the top end. Perhaps something like that is happening here.

Cheers

Ian