Feeler: Classic Solo

[ruffrecords]

I just uploaded V0.7 of the Classic Solo Build Instructions. The only change is to add the TL783 and the 1N4007 diodes to the BOM. I will leave V0.6 up there for a bit as well.

Cheers

Ian

 

[bernbrue]

Finished this cool project. Thanks a lot, Ian!

Cheers
Bernd

 

[ruffrecords]

Thanks for the pics Bernd. I have passed them on to Pierre for the gallery.

Cheers

Ian

 

[ding]

I am not going to go this route since I have a 250-250 transformer I will probably use but just in case someone wants this I drew it up. It might be more confusing than what Ian drew up but some people might think this way.

BTW Ian, I got my mouser cart today and even though you clearly have it in the schematic I somehow missed it since it wasnt in the miscellaneous BOM table and didn't get the TL783 or the 1N4007s. Might want to include it for distracted people like me.

I just noticed a couple of errors.  R22 is not fitted but is not coloured red in your drawing. Also I forget to list R26 as not fitted which you DID colour in red!! So if you would just tweak your drawing to colour in R22 red I will incorporate it into the duo doc.

Cheers

Ian

No problem.

EDIT: Here it is. You also forgot C15. Let me know if you need it in a larger size or higher dpi.

 

[ruffrecords]

No problem.

EDIT: Here it is. You also forgot C15. Let me know if you need it in a larger size or higher dpi.

You lost me. What did I forget about C15?

Cheers

Ian

 

[ding]

No problem.

EDIT: Here it is. You also forgot C15. Let me know if you need it in a larger size or higher dpi.

You lost me. What did I forget about C15?

Cheers

Ian

Sorry, in the picture you crossed out C15 on board #2 but you don't mention it in the docs. I mention it on my image.

 

[ding]

I went a little nuts with FPD and my imagination  I imagine there is very little chance of powering 2 PMTMGU with this unless I get an HT250. Got to stop myself and K.I.S.S. I might just build a separate REDD or Helios instead of this monstrosity.  ;D

 

[ruffrecords]

No problem.

EDIT: Here it is. You also forgot C15. Let me know if you need it in a larger size or higher dpi.

You lost me. What did I forget about C15?

Cheers

Ian

Sorry, in the picture you crossed out C15 on board #2 but you don't mention it in the docs. I mention it on my image.

OK, my mistake. C15 should be present on both boards which is why I don't mention it!. Can you update your picture please.

Cheers

Ian

 

[ruffrecords]

I went a little nuts with FPD and my imagination  I imagine there is very little chance of powering 2 PMTMGU with this unless I get an HT250. Got to stop myself and K.I.S.S. I might just build a separate REDD or Helios instead of this monstrosity.  ;D

If you are prepared to sacrifice a little gain you can use a passive EQ like he REDD or Helios with the Classic Solo and nothing lese. Check out the build document for details.

Cheers

Ian

 

[ruffrecords]

I went a little nuts with FPD and my imagination  I imagine there is very little chance of powering 2 PMTMGU with this unless I get an HT250. Got to stop myself and K.I.S.S. I might just build a separate REDD or Helios instead of this monstrosity.  ;D

If you are prepared to sacrifice a little gain you can use a passive EQ like he REDD or Helios with the Classic Solo and nothing lese. Check out the build document for details.

Cheers

Ian

I have been giving this some further thought, especially with a view to minimising the loss of gain by using a passive EQ and, as usual keeping costs low. Looking through some old stuff I picked up during my time at Neve I came across and article about the component parts of a mixer from 1971. it is all semiconductor stuff but it includes a simple passive two band EQ with a 6dB insertion loss (see bottom left hand corner of the attached image). It is interesting because:

1. It provides 6dB shelving boost
2. HPF/LPF style cut (no shelving)
3. Provides boost and cut with one pot
4. Can be extended to include a bell shaped mid boost/cut
5. Uses cheap linear pots
6. Could be modified to make a more ambitious/flexible EQ

The values would probably need to be modified to make it properly compatible with the Classic Solo.

Cheers

Ian

 

[ding]

I have been giving this some further thought, especially with a view to minimising the loss of gain by using a passive EQ and, as usual keeping costs low. Looking through some old stuff I picked up during my time at Neve I came across and article about the component parts of a mixer from 1971. it is all semiconductor stuff but it includes a simple passive two band EQ with a 6dB insertion loss (see bottom left hand corner of the attached image). It is interesting because:

1. It provides 6dB shelving boost
2. HPF/LPF style cut (no shelving)
3. Provides boost and cut with one pot
4. Can be extended to include a bell shaped mid boost/cut
5. Uses cheap linear pots
6. Could be modified to make a more ambitious/flexible EQ

The values would probably need to be modified to make it properly compatible with the Classic Solo.

Cheers

Ian

The more I think about it the more I want to take this on. I would be prepared to loose 15dB if need be and could just add a couple of PMTMGU in the future if I needed more gain. Going 3U will allow me to mount the power transformer at 90 degrees from the audio transformers. That would still leave me with 55dB of gain which is plenty. I have been looking around and most tube pres are between 40 and 60 dB of gain. I think 55dB should still be enough to juice up most modern ribbons if I needed them. I plan on using mainly condensers with this pre so I should be fine. The idea of a 6dB filter is a good one. A mid freq bell would be very useful.  Especially in the 400 to 6k area. 10k shelf and a 40, 80, 120 HPF would be great. For recording and general shaping 6db should be just enough.

1. I imagine to add this kind of circuit you have posted I would  need 24v or some other lower voltage to power the transistors?
never mind. Fig. 6 and 6a passive eq
2. I imagine that one classic solo with a 250v winding should be just enough to power 1 poor man's makeup? Ripple?


If I want to use just the eq with a line level source I can just plug it into the line XLR and get some of that classic duo tube vibe to boot. So I will use a 250v winding for one board with the intention of power a PMTMGU from it if needed. With the other 250v winding I will do the same for the second classic solo. I will probably do the helios since it is a little more flexible. I will attempt to wind my own inductors but that is a discussion for the helios thread.

I would be very interested to see how this 6dB eq turns out. I would love to discuss it in a design thread. Looks very interesting. Thanks so much Ian.

BTW I'm patiently waiting for your compressor design. I know you have a lot going on but is there an idea/design thread for it around?

 

[ruffrecords]

I would be very interested to see how this 6dB eq turns out. I would love to discuss it in a design thread. Looks very interesting. Thanks so much Ian.

I have done a couple of simulations. I increased the pot values to 50K so that with a bass, mid and top section there is not too much load on the driving stage. I have attached a pic of the schematic. I'll start a new Drawing Board thread for it.

BTW I'm patiently waiting for your compressor design. I know you have a lot going on but is there an idea/design thread for it around?

Funny you should mention that. You probably remember some time ago I was playing around with ideas for an opto compressor design of my own but the problem was tubes are not really designed to drive LEDs very well. In the last few days I have begun to think about a FET based compressor with all the audio going through tubes of course. There are two reasons for this; one,  I designed a FET compressor for my degree dissertation back in 1973 and two, you can drive FETs easily with tubes. I am just playing around with it right now - nothing concrete yet.

Cheers

ian

 

[ruffrecords]

I have decided to order 10 custom mains toroids. I can use two myself and I have commitments for 3 more. If you want one let me know. Cost is £22 plus shipping and Paypal fees.

Cheers

Ian

The custom mains transformers arrived yesterday. I need to check them out but if you want one drop me an email.

Cheers

Ian

 

[ruffrecords]

Inspired by this thread http://groupdiy.com/index.php?topic=62070.msg786435#msg786435 I have been running some tests on the Classic design using the Quantun QA400 audio analyser:

https://www.quantasylum.com/content/Products/QA400.aspx

This handy device allows you to look at the spectrum of an output and I have been using it to look at the noise output of the classsic design. To cut a long story short I have discovered there is some residual mains hum in the output signal. I discovered this in the output of a classic module in my Lunchbox build. I have just checked my classic solo prototype and it has exactly the same residual hum. The level of hum is very small. It is only when the gain is turned up to 70dB that the 50Hz component rises to about -50dBu. The QA400 has a A weighting option and when this is engaged the measured noise drops by about 10dB. This largest change is the hum components drop by about 25dB and I think this is the main reason for the drop in measured noise.

To verify this, I plugged the same classic module into my other test rack which has an external power supply with DC heaters.  When I measured the noise there was no hum at all (50Hz was below -90dBu) and the measured noise was about 10dB better without using the A weighting. So, having got rid of the hum due to the magnetic interaction of the mains toroid with the input and output transformers, we now have another source of hum to deal with. The evidence points to it getting in via the heaters and turning the gain down to zero drops the hum to below -80dBu so at normal settings it is completely inaudible. However, if we can get rid of it the preamp will be very low noise. The fact that it is the same level in both the classic and the classic solo PCB layouts implies it is not a PCB problem. They have different AC heater power supply PCBs but have the same level of hum so it is not the power supply design.  I am going to investigate this further to see if the hum can be eliminated without resorting to dc heaters. First thing I will do is try a traditional hum dinger pot. I'll keep you posted.

Cheers

Ian

 

[ding]

Have you tried tightly twisted wires right up to the tube pins? Are you going to feed the heater elevation into the humdinger?

 

[ruffrecords]

Have you tried tightly twisted wires right up to the tube pins? Are you going to feed the heater elevation into the humdinger?

Not tries twisted wires. They are twisted up to the point they are connected to the PCB but not from there to the tube pins. Made more complicated by having the ability to link for 12V or 6V heaters.

The heater elevation voltage reaches the heaters via a pair of 100 ohm resistors. I intend to try replacing the two fixed resistors with a pot so yes,  I will be feeding heater elevation the the humdinger.

Cheers

Ian

 

[ding]

Ian,
Any thought on a dc supply for this? I can veroboard one. Any schematics you've used for your previous projects? Best way to wire it to the board?
Thanks

 

[ruffrecords]

Ian,
Any thought on a dc supply for this? I can veroboard one. Any schematics you've used for your previous projects? Best way to wire it to the board?
Thanks

Yes I have been giving it some thought. The simplest solution is to use the existing heater winding and add a spike suppressor cap,  bridge and smoothing cap plus (small value) series resistor. With 6.3V heaters, a 4700uF/15V smoothing cap should reduce the ripple to about 1V pp which is about 20dB lower than the original ac which will bring it close to the noise floor. The series resistor is to trim the voltage to the correct value. With 6.3V heater winding you maybe will not need this because the bridge will drop 1.5 to 2V anyway. With a 12V heater winding the bridge voltage drop is proportionally smaller so you may need the series resistor. It may need to drop 4V at 0.3 amps which means it could be as high as 13 ohms. It could dissipate up to 1W of power so best to make it 5W rated.

Here is a link to a schematic :

https://drive.google.com/file/d/0B_n67A1hN3qtR0YxUjVPR21zQkk/view?usp=sharing

I might mod one of my prototypes to check this out myself. Let me know how you get on if you try it.

Cheers

Ian

 

[ding]

been looking at simple dc heater supplies for my edification and I found something curious in this article.

http://www.r-type.org/articles/art-124.htm

The article suggests coupling the dc terminals to chassis thru a couple of .5u caps to remove modulation hum in certain cases. Any thoughts? Overcautious? Should the caps short then...what? I guess this is like creating a "center tap" but with caps instead of resistors.

 

[ruffrecords]

been looking at simple dc heater supplies for my edification and I found something curious in this article.

http://www.r-type.org/articles/art-124.htm

The article suggests coupling the dc terminals to chassis thru a couple of .5u caps to remove modulation hum in certain cases. Any thoughts? Overcautious? Should the caps short then...what? I guess this is like creating a "center tap" but with caps instead of resistors.

The reason for the extra caps is because some tubes are on ac heaters and the EF86 is on dc heaters. When none of the diodes are conducting, the EF86 is no longer connected to chassis via the transformer centre tap. The caps fix this. Since all our tubes are on the same heater supply we do  not need to worry about this.

Cheers

ian