Feeler: Classic Solo

[kosi]

Hi Ian,
would a Carnhill gapped VTB2290 work as OT ?

thanks

 

[ruffrecords]

Hi Ian,
would a Carnhill gapped VTB2290 work as OT ?

thanks

It would but probably but not as well as the VRB2291. The 2281 is a gapped version which means its inductance will be lower. If you generally feed 10K bridging inputs it should be fine.

Cheers

Ian

 

[ruffrecords]

As you may know, I have been investigating residual hum in the solo and classic solo PCBs when run on ac heaters.

http://groupdiy.com/index.php?topic=62094.msg786765#msg786765

There is a clear improvement when using dc heaters although in normal use the hum is inaudible.. The Classic Solo uses ac heaters because it makes for  a simpler design. I have updated the Classic Solo PCB layout to (hopefully) reduce the hum with ac heaters and I am waiting for the new PCBs to arrive. In the meantime I thought I would try a simple mod to convert an existing Classic Solo to dc heaters. I have a 4700uF/100V cap that will fit in the enclosure so I am going to try simply rectifying and smoothing the existing ac heater winding. I will report back with before and after FFTs.

Cheers

Ian

 

[ding]

I'm still finishing up some other projects before I get to this one but I have been giving this project a bit of thought.  As I understand it the hum is negligible. Seems you need to crank the gain to 11 to reach -60ish dbfs or as you clearly put it...

the levels of hum I am measuring are very low. Typically 1mV at the output of an amplifier with 70dB of gain so referred to the input the hum level is about 0.3uV. and that is with AC heaters.

or .0003mV at 1mV output which is basically inaudible but measurable, but this is DIY so... What if I move the HT bridge rectifier away from the main board on some vero board? I'm planning on vero boarding the dc heaters so why not just add the HT bridge rectifier to that board. It seems from your tests that the main source of hum is right where the mains come in, right before the BR and its proximity to the grid of the first stage.

I would be curious to know what the FFT would look like with AC heaters and the HT rectifier away from the main board. But I guess you can just measure your Classic board for that info since the power supply is separate.

 

[ruffrecords]

I would be curious to know what the FFT would look like with AC heaters and the HT rectifier away from the main board. But I guess you can just measure your Classic board for that info since the power supply is separate.

It is not hugely different. The Classic Solo is closely based on the Classic design which is  a Eurocard sized PCB that is part of the EZTubeMixer family. The Classic solo is basically this Classic with power supplies added to the PCB. The very frust tests I did were with the Classic board and external power supplies; one with a dc heater supply and the other with ac heaters. I gave links to noise spectra of these two in the first post I made in the Drawing Board discussion of this topic:

http://groupdiy.com/index.php?topic=62094.msg786765#msg786765

Comparing these with those obtained from the Classic Solo later in the thread, it is clear that the 50 and 100Hz components are similar in both cases but the higher harmonics are at higher levels in the Classic Solo. The various changes I have made so far seem to have made most difference to the 50/100Hz components  and left the higher harmonics relatively unchanged. So you might conclude from this that moving the supply circuits away from the  amplifier has little effect on the 50/100Hz components (which are the two biggest).

Cheers

Ian

 

[kosi]

Slowly reaching the finish line !

I cannot find the value of R1 from the 4toggle pcb, either I have tomatoes on my eyes or it's simply not there.

I built the DC heater btw, thanks for the little schematic.
I'm using 12V for the heater with a 12R 5W resistor, which gives me 11V. works perfect.

 

[ruffrecords]

Slowly reaching the finish line !

I cannot find the value of R1 from the 4toggle pcb, either I have tomatoes on my eyes or it's simply not there.

There are two versions of the 4 toggle board. The first one included an RC circuit on the phantom input and I think this was originally R1. In the newer version these components are omitted which explains why there is no R1.

Cheers

Ian

 

[ruffrecords]

I am sure many of you remember I got some custom toroid mains transformers made for this project. I have had them for a quite a while and I finally got round to testing one last week. I am pleased to say it works as expected so these are now available to buy. I will not be offering these via my white market page as I am selling these at cost plus shipping. Price is £22 plus shipping/Paypal. Email me if you want one/some.

Cheers

Ian

 

[ruffrecords]

The Lo Hum version of the PCB is now available. A number are out with various people for testing evaluation so I have created a build document for this version. It is in the Classic Solo folder of the DIY section of my web site. Here is a direct link:

https://drive.google.com/file/d/0B_n67A1hN3qtYVVDTGZCVE5EZmM/view?usp=sharing

I have one prototype PCB left if anyone wants it for £14 plus shipping/Paypal. I will soon order another batch and add them to the Emporium.

By the way, all the mains transformers are gone.

Cheers

Ian

 

[ELM]

Hi there people, my first post! 
I'm one of the happy lo hum testers.
I'd just like to ad that i love the sound of this preamp! It has the sound I was looking for.
The small hum i have left in my prototype box is probably caused by badly layed out grounding and messy un shielded cables. The hum changes when i move things around. When i short pin 2 and 3 Its very quiet!
One thing that exites me about the sound is that dynamic/ribbon mics comes to life in a way that I didn't expected. 
Thanks a lot Ian!

 

[ruffrecords]

Several people have asked if it is possible to add a VU meter to the Classic Solo so I have added a section on this topic to the Build Instructions. I will also copy it and add it to the build instructions for the Lo Hum version.

Cheers

ian

 

[ruffrecords]

Hi there people, my first post! 
I'm one of the happy lo hum testers.
I'd just like to ad that i love the sound of this preamp! It has the sound I was looking for.
The small hum i have left in my prototype box is probably caused by badly layed out grounding and messy un shielded cables. The hum changes when i move things around. When i short pin 2 and 3 Its very quiet!
One thing that exites me about the sound is that dynamic/ribbon mics comes to life in a way that I didn't expected. 
Thanks a lot Ian!

It is a good idea always to use shielded cables for signals. You say it is very quiet with pins 2 and 3 shorted (I presume you mean at the input). If you just turn up the gain with nothing connected to the input it will pick up hum from the air. If you short pins 2 and 3 the only hum you get is that produced by the amp itself.

I am really pleased you like the sound. Several people have mentioned to me that the combination of ribbon mics and tube preamps makes for a very life-like sound. My first decent tape recorder was a Brenell which I got in the mid 60s. I also bought a couple of Grampiam VR2 ribbon mics, one of which I still have. I  still have some recordings of a friend playing acoustic guitar with this set up and I think it is one of the nicest sounds I have ever heard.

Cheers

Ian

 

[ELM]

Since I have the mains transformers I'm thinking of maybe rack 4 solo pcbs but then put the 4 mains transformers in a separate enclosure, just to keep the hum down.
Is there any disadvantages with that solution?

 

[ruffrecords]

Since I have the mains transformers I'm thinking of maybe rack 4 solo pcbs but then put the 4 mains transformers in a separate enclosure, just to keep the hum down.
Is there any disadvantages with that solution?

That should work fine. The power involved is small so there is no concern about voltage drops. You need to make sure the grounding and screening is managed. In particular, you really need to shield the wires from the transformer enclosure to the mic pre rack. Check out my grounding101.pdf document in the power folder of the DIY tab of my web site:

http://www.customtubeconsoles.com/diy

Cheers

Ian

 

[ejendres]

hey Ruff, I'm working on my Lo Hum build. I'm doing a dual mono setup using a single mains transformer. Reading through the documentation for Dual build i see it was written for the original circuit. In particular  am I curious about how to handle the high wattage resistors R21-R25. I assume I handle R21-24 as described in the documentation and then treat the variable resistor as R25. Is this correct?

 

[ruffrecords]

hey Ruff, I'm working on my Lo Hum build. I'm doing a dual mono setup using a single mains transformer. Reading through the documentation for Dual build i see it was written for the original circuit. In particular  am I curious about how to handle the high wattage resistors R21-R25. I assume I handle R21-24 as described in the documentation and then treat the variable resistor as R25. Is this correct?

In the Lo Hum version, the resistors R25 and R 26 are replaced by the variable resistor VR1. The instructions therefore become:

Board #1

● Build the phantom supply as normal

● Do not fit C16 and C17, R22, R23, and VR1

● Change R21 to 240 ohms 2W.

● Connect the HT, phantom and heater AC supplies to their respective connectors

Board #2

● Do not build the phantom supply.

● Do not fit BR1, C18 and C14.

● Change R21 and R22 to 240 ohms 2W

● Make the links for the heaters as normal

The connections between boards will be the same except for the heaters. For this you should connect the left and right pads of the unused VR1 on board #1 to the heater connector (or its pads) of board #2. You should use a tightly twisted pair of wire for this. VR1 on board #2 now adjusts the minimum hum point for both channels. The minimum hum point may not be the same for both boards but it should not be far away so there should be a compromise setting that is reasonably good for both. If this is not the case then, in good Dr Who tradition, reverse the polarity of the heater connection between the two boards by swapping the wires at VR1 on board #1.

Hope that helps.

Cheers

Ian

 

[ruffrecords]

I have but together a fully costed BOM for a mic pre based on the Classic Solo Lo Hum PCB. It assumes a Sowtwer input transformer (not the cheapest) and  modulshop enclosure with aluminium top and bottom covers. There are also rough estimateds for custom front and rear panels. It is a Excel spreadheet so you can download it and fill in your own specific parts. You will find it in the Classic Solo folder of the DIY tab of my web site:

http://www.customtubeconsoles.com/diy

I have also ordered another batch of the custom mains transformers. These will appear in the White Market when they arrive.

Cheers

ian

 

[Johnblue]

Ian,  I have been collecting parts for my Duo build,  and I am looking at transformers and I came across this from antek.

I am just wondering if using this would allow me to power both boards and fit all components as originally intended in a solo.

If it's pointless and overkill,  then I'll wire it the way you describe in the duo build sheets,  but if this would for some reason work better then I'll use this.  It's just 7 dollars more than the antek 240. It has two separate 250 windings for both HT and two separate heater windings. 

It's 100va 250v

 

[ELM]

Are the lo hum pcbs coming? I need more of those 

 

[ruffrecords]

Are the lo hum pcbs coming? I need more of those 

They are on order. I'll post here when they arrive.

Chhers

Ian