sontec 1 RU build thread

[Gold]

Your FI wanted 5,500 Hz for the 2nd low mid band requires a RFx of 2,323 Ohms instead of Sontec 7,987 Hz with RFx 1,600 Ohms. Increasing the pot.value to 11,129 Ohms (giving series resistance 2,323+11,129 Ohms) brings the BP center down to your wanted 950 Hz for the substituted potsCCW end instead of Sontec 100,000 Ohms for 126 Hz.The CW end of the substituted stereo pot with rev.log taper is zero ohms, setting the upper end of the BP center frequency with the remaining 2,323 ohms series resistors.

I think I'm starting to get it. To use the 11,129 series resistance calculated in the table I have to change both RF3.9 (1600Ω) and RF3.10 (1600Ω) to 2,323Ω.  To use the values from the series resistance table for the other three bands I have to change the RFx.9 and RFx.10 to the values in the top table. Lo = 4,260Ω LoMid = 14,200Ω  High = 2568Ω.

 

[Harpo]

yepp (or change cap values)

 

[Harpo]

Here is what I got empirically. Way different than the spreadsheet. I must not understand something.

Cap values for the Low band are 4.7uF / 3.3nF and 470nF / 330pF for the LowMid and HiMid band. Your measured 4.69uF parts value for the HiMid band is a decade high. Just sayin'...

 

[Gold]

Cap values for the Low band are 4.7uF / 3.3nF and 470nF / 330pF for the LowMid and HiMid band. Your measured 4.69uF parts value for the HiMid band is a decade high. Just sayin'...

It's a typo. The cap is 0.47uF. The first two measured values are in reverse order too.

I've ordered resistor values that I didn't have to make the switches. They should be here Monday.

I have a lot of 0.1% 10K resistors. Is there any reason not to change R1-R4 in the Input/Summing board to 10KΩ for improved CMRR?

Would I see better level accuracy by using 0.1% 20K Ω resistors for R18-21? The band level summing resistors. What about using 0.1% resistors for R8/R12 in the Output/Summing section?

 

[Harpo]

I have a lot of 0.1% 10K resistors. Is there any reason not to change R1-R4 in the Input/Summing board to 10KΩ for improved CMRR?

IMHO no. Good move. I'd increase the 12pF for 663kHz LPF too optimistic compensation cap value of C57 (would rise to even more insane 1.3MHz LPF with your 10K R4) to maybe 68pF for 234kHz.

Would I see better level accuracy by using 0.1% 20K Ω resistors for R18-21? The band level summing resistors.

No, (the 100Ω resistors R9 and R15 already spoil the accuracy party by factor 5) and for your wanted +/-3dB boost/cut range, these 20Ks would increase by factor 4 anyway...

What about using 0.1% resistors for R8/R12 in the Output/Summing section?

Good move and decrease both to 10K in order to half the needed parts values of the summing resistors R18, R19, R20 and R22. Parts value of compensation cap C23 as described above.
As always YMMV.

 

[Gold]

Thanks Harpo. With any luck I’ll have pictures in a couple of weeks.

 

[Gold]

It's alive! It took close to 100 hours of bench time. This is a four channel EQ for vinyl mastering. It's for the A/B path console I'm building to master directly from tape to lacquer. All the PCB's are mounted on 3/8"x1/4" aluminum stock. The aluminum stock is mounted to the sides of the chassis. That way there is access from both the top and the bottom. I cut slots in the side of the case so the PCB's could slide back and forth for better access to the terminal blocks. The slots look really rough but when faced with the choice of spending fifteen minutes with an angle grinder and over an hour on the micro mill I went with the angle grinder.

View looking through the unit.

Starting to install the switches

Switches installed

I thought having all that braided shield packed in there might have a problem shorting to stuff. But I hoped against hope it would work. No dice. Have to put clear heat shrink over all the braided shield.

Time to put it back together again.

I had two channels that I put together with pots to work through all the choices. I worked out gain, frequency and Q ranges.

I built it like I think most others. I started out with OPA604/2604 OpAmps and Wima capacitors. When I started listening I wasn't happy. The EQ sounded hard and edgy. I figured I'd try swapping OpAmps and see if that helped. I tried some LME49710/20's. It helped a little but not nearly enough to turn my frown into a smile.

After thinking about it I figured the most likely culprits were the capacitors in the tuning circuit. I pulled the Wima's. I had some other brands of 0.47uF PP caps and some 330pF polystyrene caps. I redid the two mid bands and the improvement was dramatic. I now had a smile. I ordered a few different brands of PP caps to try for the larger value. I liked the polystyrene for the smaller value so I used what I had and ordered what I didn't.

Then I hand matched all the caps to within 1%. I of course had to order 3X what I needed to sort.

Everything tested with IC's. Time to put in the DOA's. I decided to run the filter boards +/-15VDC and the In/Out/ Summing boards +/-24VDC. The console also has separate PSU's for Preview and Modulation channels. The power distribution is from the grey terminal blocks in the rear.

Knobs on.

Listening in the studio.

 

[scott2000]

Wow!
NIce!

 

[john12ax7]

What PP cap did you end up with?

 

[Gold]

What PP cap did you end up with?

The low end is a Cornell Dublier. The two mid bands are Illinois Capacitor and the high band is TDK.

I didn’t spend a lot of time on it. I think the polystyrene may be doing the heavy lifting.

The original swap was  two Wima MKS4’s for a Cornell Dublier and a polystyrene.

I wasn’t able to make the 3n3 in the low band a polystyrene until I figured out I could use three 5000pf in series parallel to get the value. The large 4.7uF made a difference but when I put the polystyrene in it helped a lot.

 

[john12ax7]

Thanks for the info.  Wima are generally pretty good but I do remember MKS to be rather bad for audio.  The MKP series should be better,  with the FKP even better,  but limited in capacitance size.

 

[Gold]

I don’t think the boards would take the footprint of an MKP. Since the leads are short you can’t shoehorn them in there. The smaller value was actually an MKP.

Edit: the smaller values were actually FKP to start with.

 

[dagoose]

wow! That's a real DIY sontec there! Cool looking panel, suits it. Congrats.  8)

 

[5v333]

applause!

 

[Harpo]

Congrats, that's some serious DIY.

 

[Gold]

Thanks guys. I don’t think I could have completed this build even a few years ago. 

I’m happy with the arrangement of controls. I oriented the controls vertically but in the horizontal order. It goes from left to right Lo Left, Lo Right, LoMid Left, LoMid Right, etc. It’s great for stereo or M/S but would be confusing for dual mono use.

I’m happy with the sound too.  I might call it YR-1 for Yaht Rock. It has that soft focus Super 8 kind of sound. That’s exactly what I was hoping to get out of it.

I’m working on the second unit for the console now. The only thing I’m going to change is some of the terminal blocks. I thought there would be better screwdriver access from the rear so I got angled blocks. It didn’t work very well so I’m using regular 90 degree blocks.

I put some shots of me making the faceplate in the Machine Shop.

 

[Gold]

I had a happy accident when building this. The Electroswitch C4's have D shafts. The OKW knobs are collett mount. I've had both the switches and the knobs for years. If you just try to tighten the colletts on the shaft they slip. My first thought was to get some 1/4" half round stock to make the D into an O. The flat of the shaft doesn't take up 50% of the shaft diameter so it's not a drop in solution. The next thought was to get some thin wall brass tubing and glue it over the shaft. That didn't work either because no tube wall was thin enough to get the collett over.

When testing this EQ out I didn't want to take the knobs on and off a million times so I used a pliers with teeth to rotate the switches. It gave me a good grip but tore up the switch shafts. I figured I could smooth them out later if necessary. When I went to put on the knobs I discovered I had inadvertently made a lock washer. The colletts were snug over the roughed up shafts and don't move. Problem solved!

 

[Gold]

This is Unit B. I ended up spending about as much time on this one as the previous one. I was able to tighten things up a bit. The frequency centers on Unit A were tightly matched between channels but didn't match the faceplate markings as closely as I'd like. On this unit I was able to trim them so they come pretty close to the faceplate markings and are still tightly matched. I had to trim the polystyrene caps which wasn't easy as there aren't too many values available anymore.

I put relay bypass boards in this one but didn't connect them. If I decide I want that in the future it won't be too difficult to install them. I forgot the LED's need a switch pole. The pushbutton switches are only 2 pole so I can only switch audio and not any LED's. I think I'm going to go with a 6 pole F style pushbutton switch so I can switch everything at once.

On the A Unit I used 31mm knobs for Frequency Select because they are 4 deck switches. These Electroswitch C4's are stiff. That didn't leave enough room for the legend. On this one I used 23mm knobs for everything. I think it looks better and the switch torque doesn't seem too much different than the others. Now I'll go back to Unit A and clean some things up.

 

[Script]

Found hard bypass boards meant for Igor's Sontec (red boards v1.7). Are these bypass boards actually by Igor? Anyone has documentation on them ?

- Relays can be wired in parallel or in series (depending on voltage).
- There's a ground filter (10R and C.1)
- CZ, RZ and R_L are Zobel Network for optional output TXs ?
- Vrel and Vrel1 is for hooking power to a second bypass boards
- BYP_SW activates relays or LED

 

[Harpo]

Found hard bypass boards meant for Igor's Sontec (red boards v1.7). Are these bypass boards actually by Igor? Anyone has documentation on them ?