Feeler: Classic Solo

[ruffrecords]

Nice docs as always! Very thorough. I like all the options you built into this. HPF and continuous input pad are great. The SW PCB is really nice. Its going to be a fun build. A few questions...

1. To build as a pre with DI the DI input would go into J5 and J4 would be shorted? Or would it be plugged into J4 like the DI box? Or either? On the PCB it looks like J5 is going into the TX.

If you want a mic pre that also has a DI input you wire the DI to J4 just like the DI box. The mic input goes into J1 as usual. The transforme routput from J5 then needs to be wired to the DI input socket. You need to arrange the socket contacts to connect the transformer through when nothing is plugged into the DI input and to disonnect it ehn the DI is plugged in. I'll add a schematic of this to the docs.

2. Can the "Drive" pot be connected to J4 even if I am building the pre just to get some dirty drive when required?

In the mic pre, the transformer output goes straight into the tube input - it is as if the drive control was at 100% all the time. Drive is adjusted by the pad before the mic input transformer. The variable pad is effectively your drive control for the mic pre. When you drive the first stage hard, its output can be very large - as much as +30dBu. The output stage has about 22dBu of gain so, for a nominal +4dBu final output, the input to the last stage needs to be about -18dBu. This means the gain pot needs to drop +30dBu down to -18dBu, a total of 48dB. Under these circumstances, the gain pot ends up being very nearly turned completely off and adjustment is very  touchy. In a previous deign, to combat this problem, I added an extra switched 20dB pad just before the gain control so the pot can be used  near the middle of its range. This makes no difference to the amount of drive you get. It just manages the downstream gain. I'll add this to the docs as well.

3. If I wanted to add the REDD EQ or any of your other EQs would it be before the 10k gain pot like the HPF setup?

You should add these EQs after the 10K pot. You can still have the HPF before the pot as well if you wish. I'll add this to the docs as well.

4. I know this will mess with the impedance but is it possible to add an output attenuator after the output transformer in the style of the SR124 or in the style of the attached image?
This question is more about wanting to drive the hell of of this preamp if need be and get a little tube dirt for example on a DI bass. I know that this is a clean tube pre design and that you have taken measures to get a high gain, clean sound but options are always good. I guess if a "Drive" pot is possible than this question is moot anyways.

Yes you can add an output attenuator after the output transformer which will allow you to drive the second stage harder as well. You can make a simple one with a pot and a couple of resistors:

https://drive.google.com/file/d/0B_n67A1hN3qtVTVZaUR3blFMYWs/view?usp=sharing

5. If I were to build this in a dual configuration is there a simple way to attach power, 48v and heater from 1 board to the other so as not to build 2 power supplies?

Heaters are easy. They are AC so just parallel up the connection from the mains transformer. The phantom power supply should easily supply a couple of inputs so just build one of them and wire it to both mic inputs. The problem is the HT because it also creates the heater elevation voltage. The HT supply is designed to run one channel alone, so as well as using lower value smoothing caps I removed one stage of smoothing, to save both space and money. Ideally you want one HT supply to power two boards. To do this just reduce the value of R21 and R22 to 470 ohms. Connect the HT from the junction of R22/R23 on one board to the same point on the other. Link the AGNDs of the two boards together and connect the AGND of the board with the power supply to the chassis bolt. I'll probably add this to the docs too!!

6. Again messing with impedance but if a meter is required it would go after the output TX?

After the TX, yes

I really like stepped gain pots so I would probably build a stepped 10k switch for the gain. Something like http://diy-tubes.com/gain-switch-kit.html or P2P

Thanks,
Alain

Stepped gain controls are fine. The stepped pot you linked to is not a stepped pot - it is a stepped series resistor. However, you can probably solder an additional wire to the PCB to make it into a proper pot.

Cheers

Ian

 

[ding]

Stepped gain controls are fine. The stepped pot you linked to is not a stepped pot - it is a stepped series resistor. However, you can probably solder an additional wire to the PCB to make it into a proper pot.

Cheers

Ian

Thanks Ian. If the gain control works like you are saying than I think a stepped pot would be unwise. I might need the extra fine tuning control of a regular pot.

 

[ruffrecords]

Stepped gain controls are fine. The stepped pot you linked to is not a stepped pot - it is a stepped series resistor. However, you can probably solder an additional wire to the PCB to make it into a proper pot.

Cheers

Ian

Thanks Ian. If the gain control works like you are saying than I think a stepped pot would be unwise. I might need the extra fine tuning control of a regular pot.

I think you are right. It is more like a fader than a stepped gain control.

Cheers

Ian

 

[bernbrue]

Hi Ian,
Thanks a lot for the nice reader. The psu schematic in your reader shows that the heater voltage is derived from high voltage whereas your description of suitable power transformers mentions that a 12V/6,3V secondary is needed.  That's a bit confusing. I'm shure this is a leftover from your classic preamp reader.  I'm impressed by the versatility of the preamp. Nice concept.
Best
Bernd

 

[bernbrue]

Sorry, ignore my comment. Heater is AC. Let's go on.
Bernd

 

[ruffrecords]

I have updated the Build document to include answers to most of the questions raised by ding. I have omitted  powering two boards from the PSU of one board. I will create a separate document for this. Updated doc is in the Classic Solo folder of the DIY tab of my web site. Direct link here:

https://drive.google.com/file/d/0B_n67A1hN3qtN0hLZFYzcWI2WFU/view?usp=sharing

Cheers

Ian

 

[ding]

I think I'm just going to build out the extra HT. it's not like it's going to break the bank. Probably just use one 48v supply though.

 

[ruffrecords]

I think I'm just going to build out the extra HT. it's not like it's going to break the bank. Probably just use one 48v supply though.

That would simplify things a lot. It avoids possible ground loop problems. With two HT supplies you just wire the AGND of each one direct to the chassis bolt. One word of warning.  If you are sharing AC heaters, which it seems sensible to do, you could end up with two heater elevation circuits fighting each other.It should not really be a problem but just to be on the safe side you should fit the heater elevation components only to one of the HT supplies. On the other hand, if you manage to get hold of a mains transformer with two separate  AC heater windings, and there are plenty of them about, you can  build both HT supplies as normal and just wire AC heater windings,the PCB to separate windings.

Cheers

Ian

 

[ding]

That crazy big PTX I got looks like it has 2-250v at 0.2A and 2-6.3v at 3A so I will just run the heaters for each board @6.3V with no problem. Where is pad "D"? I'm looking at the PCB on the PDF and I can't seem to find it. Also fordable 50v transformers are a bit of a pain to find, can I modify to use 48v TX instead? http://catalog.triadmagnetics.com/Asset/VPT48-1040.pdf

 

[ruffrecords]

Pad 'D' is just inside the bottom of the V1 circle. I have uploaded a copy of the PCB laypout pic to the Classic Solo folder of my web site. Direct link here:

https://drive.google.com/file/d/0B_n67A1hN3qtRWN1ZUtNSk9sQm8/view?usp=sharing

For the 50VAC you need very little power - you really need no more than a 5VA transformer even for two channels. Because the power consupltion is so small, you can use almost any transformer with 2x24V or  2x25V secondaries. There are lots to choose from, Here is a 15VA one from Farnell:

http://uk.farnell.com/multicomp/mcfe015-25/transformer-15va-2-x-25v/dp/9531696

You don't have to use a toroid as long as you keep it away from the input transformers. You could probably use one of these:

http://uk.farnell.com/block/fl4-24/transformer-4va-2-x-24v/dp/1131532

hders

Ian

 

[ding]

Looks like hairball also has one. Probably be better on shipping for US folks.
http://www.hairballaudio.com/catalog/parts-store/power-transformers/dual-primary-toroidal-2x25v-30va-power-transformer

 

[ruffrecords]

Looks like hairball also has one. Probably be better on shipping for US folks.
http://www.hairballaudio.com/catalog/parts-store/power-transformers/dual-primary-toroidal-2x25v-30va-power-transformer

Don't forget Farnell is Newark in the US. I believe they use the same catalogue numbers.

http://www.newark.com/multicomp/mcfe015-25/toroidal-transformer-15-va-2-x/dp/38K4544?ost=9531696&selectedCategoryId=&categoryName=All+Categories&categoryNameResp=All+Categories

http://www.newark.com/block/avb-3-2-2-24/transformer-iso-48vac-3-2va/dp/53X8390

Cheers

Ian

 

[Rocinante]

Yeah, I know its overkill but it's what I have at hand. I was thinking of getting a second small transformer for 48v. Maybe I'll build it into an external 1U PS to power 6 of these  One could only dream.  :'( I'm also considering a 1 Solo+1 REDD EQ 2U channel that might be nice.  I imagine I would also need the PMTubeMakeup if I was going that rout. I guess for the extra makeup gain I don't need extra transformers, is that right? It will all come down to Iron and how big of a

By the way, I am working on new version of all three of my passive EQs with built in gain make up amplifiers. I have just completed and am about to test the Pultec version. REDD and Helios will follow in due course.

Cheers

Ian

Will these have the same gain stage as your other projects ( pmtgmu and classic) or are you brewing something different?
I am having a hard time picturing the helios or red filters sharing the same pcb with the pmtgmu.
Needless to say I'm intrigued.

 

[ruffrecords]

Will these have the same gain stage as your other projects ( pmtgmu and classic) or are you brewing something different?
I am having a hard time picturing the helios or red filters sharing the same pcb with the pmtgmu.
Needless to say I'm intrigued.

It is the same basic topology, a mu follower, but it uses a 12AU7 tube. Theres a pic of the Pultec version PCB in the EQwithgainmakeup folder of the DIY section of my web site. Direct link here:

https://drive.google.com/file/d/0B_n67A1hN3qtdVFsSFlLRTFrbU0/view?usp=sharing

It is a standard Eurocard size PCB (160 x 100mm) and is fully compatible with the EZTube Mixer system and LunchBox. There's even room for an input transformer.

Cheers

IAn

 

[Rocinante]

Ahh!!!I cant I can't I can't.  But it looks so tempting....
Something to consider; Ive seen designs where the tube lays sideways via a second external board that gets tied in. Sound sculptures tube preamp and gix 51x come to mind. It would be neat for maybe a thinner module? Could come attached and get cut out of the pcb.
I'm not saying you should revamp your design; its awesome. Just throwing ideas out there. Might cause noise though being so close. At any rate looks awesome Ian and that its 12au7 makes it even easier.

 

[ruffrecords]

I have  several times looked at mounting tubes vertically to make modules thinner, especially in the days when I was using 6CG7 tubes which are even taller than the 12AU7 and friends. However, I have always founds that this brings more problems than it solves. For a slim module and a robust connection to the tubes it helps to mount the tube as close to the PCB surface as possible but this means all that PCB space can no longer be used to mount components, not to mention the effect the heat from the tube would have on those components if you could squeeze them in. The current standard modules are 5.25 inches high (3U) and 2.8 inches wide. By the time you take out the space at the top and bottom to mount the unit in a rack and the space for mounting the PCB to the front panel and a screening enclosure, there is surprisingly little front panel space left. My 3 band Pultec EQ for example needs three rotary switches, two toggle switches and and six pots on the front panel which is very tight.

Despite these reservations, or perhaps because of them, I am working on a range of twin channel 6U line level modules. This means you can get 12 channels in a standard rack instead of just 6. The extra height means there is more room to lay out the EQ controls and there is room at the top to include pan and AUX send controls to turn these into general purpose line level channel modules. Ideal for mix dwon and with external mic pres it makes a general purpose mixer.

Cheers

Ian

Cheers

Ian

 

[DerEber]

Hey Ian,

I´m in for a Stereo Classic Solo.

best,
Stephan

 

[erikb1971]

Am I the only one dreaming of a portable classic solo with di in a portable housing like an ISA one?

 

[ding]

Am I the only one dreaming of a portable classic solo with di in a portable housing like an ISA one?

I think that is Ian's intended purpose for this but you know, if one is good...  ;D

 

[ruffrecords]

Am I the only one dreaming of a portable classic solo with di in a portable housing like an ISA one?

No reason not to have one. I have tried it in a Hammond enclosure and a modushop enclosure so far. I think there is a Hammond enclosure a lot like the ISA but it was rather expensive so I did not pursue it.. Just to give you an idea of overall size, the modushop enclosure is 2U high, about 230 mm wide and 230mm deep. The idea of the Classic Solo is that it is completely self contained - just plug it into the mains and off you go.

Cheers

Ian