Overkill tube pre/EQ called Drive-1. Maintenance update.

Here's what I've been working on lately.

At heart it's a "one bottle" 6SN7 preamp with a unity gain treble and bass tone stack between the 6SN7 input and output stages. Then there's a whole separate line amp as the output stage. I have already used this line amp in several other projects and basically don't want to live without it anymore. If you don't need the gain the line amp part can be considered optional, but it will also happily drive a 2:1 output transformer and sounds wonderful when overdriven. The V1B stage alone (6SN7 or variant) used as output will need at least 5:1 ratio.

There are just about no tricks in this, as any tube guy will quickly see. I don't intend to cover any new ground here. I'm just gluing together some proven solutions I like, and as such I don't really deserve any credit for it. Advanced copy and paste with my own preferences for tube bias. This is a draft and I will end up tuning bias, pads and tone stack for best sounding distortion points when I hear them first.

The design is absolutely loaded with features and switches, most meant to be relays to minimise hum when I start laying out the thing. You can downgrade performance in several ways and the unit is basically a template for experimentation with tube distortion. There's a possibility of feedback, no feedback, over-bias, under-bias, cathode bypass, no cathode bypass, starved plate and what else! At optimal settings it will be a turbo-charged clean preamp with plenty of headroom. When all options are switched to "completely wrong" it makes various types of distortion/overdrive. Will work great for line level sources, and that's what I will be mostly using it on. I have intentionally biased the optimal settings a bit on the high current side at the expense of raw gain and voltage swing. Tubes seem to sound better this way.

At unbypassed setting max gain is around 75dB (with no output stage feedback).
Cathode bypassed setting max gain is about 90dB (with no output stage feedback).

PSU is a brute force regulated affair with far too much filtering and voltage dropping just in case. This is supposed to be a very wasteful current eater, I don't care.

If you spot errors that will downgrade the design, or have more ideas, now is a good time to mention about them.

I already have a question on the use of feedback. Do I need to add a cap between C7 and R12/R13? Will the R16 somehow screw up (bias?) the tone stack feedback?

What about the output stage feedback? Do I need a similar cap there, or is C12 (connected to output trafo) just fine already?

Thanks,
Mike

[edit]
revision 1.9 with many fixes and various tweaks. There used to be NFD loop around the last two stages, but it was completely pointless. The design does 0.1-0.3% THD+N even without it - mostly second order harmonics, invisible. The option is still there on the PCB. PCB's have seen various levels of development and prototyping too, and these seem to work well, giving the builder plenty of options for further experimentation. The "plate starve" option of the PSU that drops B+ voltage is of the dumb-but-works school of design. It wastes a stupid amount of watts and needs a big-watt resistor, but whatever. The EQ frequency selectors, including the caps are probably best wired directly on rotary switches. The relays limit the choices to only two frequencies per bass and treble control. nF and pF range caps are tiny so this is easy to do.

initial working and well tested release 26.2.2012
http://www.michaelkingston.fi/files/Drive-1_schematic_rev1.9.pdf
http://www.michaelkingston.fi/files/Drive-1_PSU_PCB_1.3.pdf
http://www.michaelkingston.fi/files/Drive-1_overdriving_amplifier_PCB_1.1.pdf

maintenance update 3.6.2013
Note, the above overdriving_amplifier_PCB_1.1 is not perfectly compatible with the below update. PSU was not changed. see this post,
http://www.groupdiy.com/index.php?topic=41509.msg672213#msg672213

http://www.michaelkingston.fi/files/Drive-1_schematic_rev2.0.pdf

maintenance update 21.1.2014
Heater Regulator removed. see this post,
http://groupdiy.com/index.php?topic=41509.msg701614#msg701614

http://www.michaelkingston.fi/files/Drive-1_schematic_rev2.1.pdf





For reference here are some relevant threads that led to this design.
http://www.groupdiy.com/index.php?topic=41313.0
http://www.groupdiy.com/index.php?topic=41412.0
http://www.groupdiy.com/index.php?topic=41371.0

Kingston said:

I already have a question on the use of feedback. Do I need to add a cap between C7 and R12/R13? Will the R16 somehow screw up (bias?) the tone stack feedback?

Click to expand...

No

Kingston said:

What about the output stage feedback? Do I need a similar cap there, or is C12 (connected to output trafo) just fine already?

Click to expand...

It's fine.  Insertion of cap is a way to compensate EQ towards additional bass, if needed.  That's the only time you see series caps in one path of a fork. 

Great to finally see this thing!

What do you like about that line stage?

Thanks emrr for the advice. I already had good confidence on that plan, but had to make sure. Think I'm ready to start laying this out. Still contemplating over whether I want an "armored PCB" a la drip electronics, or plain p2p.

eskimo said:

What do you like about that line stage?

Click to expand...

6N6P is a superb sounding tube and I use any excuse I have to put it in signal path. Headroom for miles, or great invisible distortion. Especially with well balanced (frequency-wise) material you can just keep pushing and pushing it to more distortion and up to a surprisingly high point (at least 2% THD) it will only sound better. PS. make sure output transformer has ample headroom. It's likely you will clip it before ECC99/6N6P gives in.

That one combined with another flavor of invisi-distortion TM [all rights reserved; patent pending]   from 6SN7 or variants, and I think I finally have the shaper I have always wanted.

Also notice the list of tube alternatives I provided. It is possible to build a real ghetto version with 12AU7 and 12AX7 and edcors.

The only downer is the 750 mA, ouch!

Very Nice! 

Couple of things:

1. I see no phantom power.

2. The 6.3V dc supply is a bit of an overkill starting out as it does at 16.8V.

3. The output stage gain control will not work as it is INSIDE the NFB loop. It should be where R19 is now.

4. You do not need the GS on the grid of V1A - it will just add noise and the gm of the 6SN7 is low enough for it not to be necessary (and for the same reason you do not need it on V1B).

5. You do not need the GS on V2A - it's a cathode follower.

6. Why are you loading the output transformer with 610R??

7. There's no point R10 being greater than 100K - you do not have enough open loop gain for anything higher.

Cheers

Ian

Forgot to ask, what program did you use to draw the circuit? - very professional looking.

Cheers

Ian

Thanks for the input.

1. I nearly never use phantom powered mics and I have plenty of preamps with phantom already. The utilities part of PSU is there if someone needs it. A voltage multiplier with a regulator could be added, but I have no use for it.

2. Voltage wasted there with lots of heat, unfortunately. But I wanted to use easily available transformers, and this combo had least other issues. Next option would be 115+115:9+9 transformer to 6+6:115+115 but big 6 volt transformers were not available to me. A custom transformer would be the best option. Here's one for a dual channel unit if someone is interested:

primary: 115+115VAC
secondary 1: 300VAC 200mA
secondary 2: 9VAC 5A
secondary 3: 12-15VAC 200-500mA (optional for utilities like relays and leds)
secondary 4: 50VAC 50-100mA (optional for regulated phantom)

3. Feedback loop is behind a switch (or could be a rotary). Running the stages with no feedback is probably the most desirable mode. I will use only 10dB feedback max anyway. Moving VR4 to the R19 position defeats the purpose of being able to heavily overdrive V1B and V3A together. Distorting those two stages (fine tuned with the tone stack) is pretty much the main reason I'm building this thing. VR4 takes that LOUD distorted sound to sane levels for V3B and output transformer, even with mild feedback around the output stages. Oh, and I can separately clip the output transformer, and again take sound to sane levels with output pad.

4. and 5. Mentioned in the initial post above, all the GS resistors are for trouble shooting only. Most likely not needed. Cathode followers apparently need them sometimes as well. I read that somewhere in tubecad website, but could not find the link just now.

6. It's from Cinemag CM-9589 datasheet. Can be ignored. Many other high headroom 4:1 to 2:1 ratio output transformers work here as well and would probably like to see a different load.

7. I see. The tone stack and 12AX7 around it are from here: http://www.angelfire.com/electronic/funwithtubes/Amp-Tone-A.html

(Oh and I should add the info to the schematic, credit the original author)

Would think a 12AX7 (or other very high-mu triode) has enough open loop gain? I will experiment with this, but the graph from that website says it works well.

The schematic was drawn in plain corel draw (vectors). I have made a kind of a library for different parts I can re-use everywhere. It's manual work, no auto-routing or anything like that, but beats a pencil and a sketchpad. I do all my PCB's and front panels there as well.

ruffrecords said:

3. The output stage gain control will not work as it is INSIDE the NFB loop. It should be where R19 is now.

Click to expand...

Gates SA-70 does this, yes it's non-standard, but works fine in practice with lower feedback amounts. 

Hi again:

Couple more points!!

3. The SA-70 does not do this! It is a completely different sort of NFB loop.

7. You seem to have two different resistors labelled R10 - one in the tone stack and one in the output stage. I was refereing to the one in the output stage LOL.

Cheers

Ian

The SA-70 does not do this! It is a completely different sort of NFB loop

Click to expand...

Are you referring to a difference between plate to plate vs plate to cathode FB?


Since this project is in the midnight hour - I make a strong vote for building the circuit on a test board first.  Especially since the amps purpose is centered around the art of pleasant sounding HD.  Ear tweaks vs simulators and any other unforeseens.

Open loop gain figure for the entire circuit you're getting is?

SW3 - this will cause very loud pop when you flip it.

ruffrecords said:

3. The SA-70 does not do this! It is a completely different sort of NFB loop.

Click to expand...

It does in that there is a voltage divider pot within a feedback loop; my only point.  Care to spell out the differences between the two FB variations?   I think it not so great in practice, especially at 10 dB max.  I just re-read an article about NFB written by a Gates engineer, which explores 6 different NFB methods in the context of the SA-70 circuit.  They do not elaborate, beyond showing all the different ways that will work. 

ruffrecords said:

7. You seem to have two different resistors labelled R10 - one in the tone stack and one in the output stage. I was refereing to the one in the output stage LOL.

Click to expand...

whoops! a typo slipped in. That's what you get for manually drawing stuff in corel. I'll fix that to the next revision.

About the debate over the attenuator inside the NFB, like I said I've built this line amp already, the only difference this time is I don't have a separate NFB cap, but shared one with output transformer coupling. The NFB resistor is a 500k trimmer. I seem to recall I set it at around 200k for a passiveEQ make up gain stage, which got me low THD at less than 10dB NFB. Didn't seem to have much effect on the attenuator.

I saw Gates SA-70 just now. I don't quite understand the feedback there, seem to be sent to 6J7 grid and plate both?

But I know the difference between sending feedback to cathode, or to the grid (like in single stage plate follower with feedback) is that cathode input impedance is much lower. Non-issue with fat triodes like ECC99.

lassoharp said:

Since this project is in the midnight hour - I make a strong vote for building the circuit on a test board first.  Especially since the amps purpose is centered around the art of pleasant sounding HD.  Ear tweaks vs simulators and any other unforeseens.

Click to expand...

This is the test board. I've covered nearly all the options related to dialing the perfect distortion. Apart from the tone stack and those zener regulators I've built the other circuits of the plan in various projects and they work well.

I already know at worst it's going to be a really good sounding clean preamp. The "unforeseen" part here is the fun I'm looking to have tuning out the last little bits and experimenting with overdrive, dirtying up the optimal settings.

Think about it, there's a one bottle 6SN7/12AU7 preamp with no feedback similar to NYD plans years a go, a clean "unity gain" tone stack, and a strong line amp with a very stiff PSU. What could possibly go wrong?

lassoharp said:

Open loop gain figure for the entire circuit

Click to expand...

6SN7/6N8S mu ~21, rp 7K2-7K7
V1: 170V, plate resistor 27K(.6W), cathode resistor 1K8, 2.2mA/-4V, unbypassed: Av 7.7, 18dB,
V2: 250V, plate resistor 15K(2 W), cathode resistor 1K2, 5mA/-6V, unbypassed: Av 6.3, 16dB, bypassed: Av, 13.9, 23dB

12AX7/6SL7/6N9S mu ~80-100, rp 50k-70k
V1: 250V, plate resistor 18K(.6W), cathode resistor 1K8, 1.1mA/-2V, as cathode follower
V2: 250V, plate resistor 100K(.6W), cathode resistor 1K8, 1.1mA/-2V, bypassed: Av, 52.1, 34dB
(gain is about -1 dB for the whole 12AX7 active EQ stages together)

ECC99/6N6P mu ~21, rp 1K75
V1: 250V, plate resistor 24K(3W), cathode resistor 1200, 5mA/-6V, unbypassed: Av 9.7, 19.7dB
V2: 300V, plate resistor 15K(5W), cathode resistor 620, 10mA/-6V, unbypassed: Av 8.6, 18.7dB, bypassed: Av, 17.9, 25dB

The whole circuit with transformers included:

At unbypassed setting max gain is around 75dB (with no output stage feedback).
Cathode bypassed setting max gain is about 90dB (with no output stage feedback).

VacuumVoodoo said:

SW3 - this will cause very loud pop when you flip it.

Click to expand...

The feedback switch also pops quite nasty especially with lots of feedback, same with the bass freq cap if the switch is not MBB. If some of the switches of the plan turn out dangerous or useless, they will not appear on the front panel.

Think I'd be using separate switches for SW3, rather than linked.   If you're gonna go there, go all the way.  

6. Why are you loading the output transformer with 610R??

Click to expand...

Pad and increased harmonics generation would be 1 why.  I like 680R personally, when it's called for.  Better yet, determine ideal through bench measurements.  I found a vast difference within the RCA modules I recently finished. 

Think about it, there's a one bottle 6SN7/12AU7 preamp with no feedback similar to NYD plans years a go, a clean "unity gain" tone stack, and a strong line amp with a very stiff PSU. What could possibly go wrong?

Click to expand...

;D ;D   You will only know once it's fully built.  Entropy rules.    

SA-70 uses straightforward plate to plate FB variations.  

Feeding 12VAC into a 9VAC winding is a bad idea. An iron-core winding is a high impedance to some AC voltage, then it goes low impedance. Commercial ratings have some margin of over-voltage, but often not a lot. With small E-I cores the soft-knee and copper resistance avoids a smoke-down, at the 100VA level you may have large saturation current and heat.

I'd try this part outdoors before drilling a chassis.

I see.

I've done a lot of 15VAC to 12VAC for 300VDC B+ lines in the past, always toroids. I once smoked the 15VAC one in that configuration, well not so much smoked, but it stopped working and never woke up. But that one was because of way too low rating on the 15VAC winding for 12.6VDC heaters. Fuse was too big so the transformer decided to act as one. Of course I had already drilled the chassis and the bigger one did not gracefully fit in.

Yes, 230VAC winding tricked into working as 300VAC is probably not the best of ideas, over voltage-wise. But never had a problem there in the past. What's the worst that could happen? Internal arc in the transformer and a dead B+ line? I know some people who work at trafox, a Finnish manufacturer of very high quality toroids I often use. I'll ask them about the over voltage issue.

I rated the 9VAC 50VA transformer to about triple of what is needed. 50VA / 307V = 163mA for the high voltage part. I need about 50mA for a dual channel unit. Good buffer for current at least.

[edit]

just checked with trafox,

they hi-pot test their standard model toroids with:
primary - secondary 4000V
secondary - secondary 4000V

They didn't say at what current, but the above is at least an indication that my plan is reasonably safe.

[edit for an edit]

and now I was further corrected that the actual windings will not take more than about 10% over voltage. The above 4kV is for insulation only! I was told the windings themselves will "pop the fuse right off the wall" when used in severe over voltage condition.  

The configuration has worked ok for me in the past, but I guess if even trafox advices against it, it's time to stop doing it.

Well dammit, looks like my plan just got significantly more expensive, unless somebody knows a cheap and easily available 230VAC to 300VAC 100-200mA transformer. There's always hammond, but I hate to pay for the mandatory 6.3VAC windings and then throw them away. And before somebody suggests it, I don't want to gamble with VAC heaters in a preamp with this many stages.

In other news, the massive heat and wasted voltage in 12VAC to 6.3VDC for heaters will be solved at least, since the series transformer trick is now scrapped.

I will draw a new revision of the schematic with a custom transformer marked in later today (the one I already suggested earlier in the thread).

you could use a 400V/12V transformer...
http://fr.farnell.com/block/steu500-24/transformateur-500va-230-400v/dp/1131587

a little more than 100 euros with VAT for 500VA though...

A custom 750VA toroid with 2X 115 primaries, and 4 secondaries would cost you 153 euros including VAT here:
http://www.audiophonics.fr/transformateur-torique-sur-mesure-toutes-puissances-p-3675.html

And only 130 euros if you get enough of 630VA...

Axel

500VA! and it weights 7.7kg! That's one serious safety margin. I suppose passive burglar protection as well.

Realistically, the one I want is on this list:

http://uraltone.com/kauppa/index.php?cat=c35_Power-transformers.html&XTCsid=j69a6f9c9nt9c0rr7461oi1al5

one from Uraltone power transformers, or something from Hammond 260-series.

It still feels wrong to waste a perfectly good 6.3VAC winding, but if that's how the PSU trafo manufacturers want to play...