API 325 to 312...help please.

[drpat]

...

 

[furmound]

-All R3's on my 325-4's say 47K on them. Will the 50K reverse taper pot work?

-I know there's different kinds of capacitors and resistors. Obviously I have to match values but other than that should I look for something specific? Gosh, what a nube question.

Thanks for making this possible.

 

[peter purpose]

50K will be fine if you leave 47K resistors in there.
I use Panasonic FC for the electrolytics (big cans) and ceramics of any type for the small value jobbies.

peter

 

[peter purpose]

 

[peter purpose]

1/4 watt metal film for the resistors.

peter

 

[furmound]

oops

 

[furmound]

Great! Thank you very much sir.

So, after making all caps Panasonic FC except for the smaller ceramics and all my resistors 1/4 watt metal film this is what my order with digikey looks like.

1-(10) EDC306150-ND CONN CARDEDGE 15POS .156 X .156   0 2.44800 $24.48

2-CR1& CR2-(18) 1N4007DICT-ND RECTIFIER GPP 1000V 1A DO-41 0 0.13600 $2.45

3-C1 & C2-(18) P10326-ND CAP 220UF 50V ELECT FC RADIAL 0 0.54500 $9.81

4-C3-(10) P10253-ND CAP 1000UF 16V ELECT FC RADIAL 0 0.50400 $5.04

5-C5-(10) P11414CT-ND CAP 220PF 250VAC CERAMIC Y2/X1 0 0.35600 $3.56

6-C6-(10) PCC121ACVCT-ND CAP CERAMIC 120PF 50V 0603 SMD 0 0.04400 $0.44

7-R4-(10) 311-5.1KECT-ND RES 5.1K OHM 1/4W 5% 1206 SMD 0 0.07800 $0.78

8-R1-(10) P100KCACT-ND RES METAL FILM 100K OHM 1/4W 1% 0 0.17100 $1.71

Subtotal $48.27

The only thing that looks strange to my inexperienced eyes is C5. It's specifically rated at 250V AC. Is that the wrong one? Everything else is rated so much lower and AC is not mentioned.

I'm also ordering the reverse pots from vibroworld.

BTW, i'm might be racking 8 of these instead of 2 which is why i'm buying larger quantities.

Thanks again.

 

[soundguy]

If you use panasonic FC caps, your amps might sound great to your ear but will have a very different midrange quality to them compared to what those things sounded like when new. Digikey sells BC caps which are basically the same caps that API originally stuck in there. The FC's will completely outperform the BC's, but if you want it to sound "classic" you might consider ordering both and listening to each to see which you prefer.

dave

 

[furmound]

If I can change my order I'll buy the BC's as well. I'm interested in a variety of sounds so I'll probably keep a combination of both.

Thanks.

 

[drpat]

...

 

[furmound]

Wow! I had no idea that waiting for parts would be so painful. I find myself gripping on to my soldering iron while staring at my 325 cards with anticipation. In the meantime I was wondering if I could use some other input and output transformers with these 325 cards for sound variety. I have a couple extra Yamaha pm-1000 and Ward beck input and output transformers. They are:
yamaha-
in- GA80071
out- GA80080

ward beck-
in - "HEJ89" "60-12" "113661" "DBOE"

Does anyone know the specs for these? If not, how do I find out myself? (I've tried to google them and I don't know how to test transformers) Is it possible (or even worth it) to use these with the 325 cards to make mic pre's?

Thanks.

 

[furmound]

Hello again. While repopulating my 325 cards to make them into pre's I noticed that some of the components in the pictures that I've seen do not follow the schematics very strictly.

For example, the first thing the pins 4 and 5 on the 325 cards is C5, but it seems to me that on Peter Purpose's pictures that slot was replaced with R1. I did the same thing. Is this OK?!?
Also, I'm supposed to replace C5 with the correct termination for the 2622 transformer which is the 220pF in series with the 5K ohm. According to the 312 schematic the resistor is the closest component to the plus side of the 2520 opamp but on Peter's pics it seems like (it's hard to see because of the shadow) the resistor is first and the cap is second. Does the order of these matter?

Thanks for all the info and guidance Peter, I just want to make sure I'm interpreting the information correctly.

I'm also unclear regarding my transformer connections because I do not have much info on the 2622. It seems to have two primaries and a secondary. One primary is labeled hi and the other lo. Is this lo impedance and hi impedance or is it refering to possitive and neg?
If it is impedance can I attach a 1/4" connector to the hi z one for that kind of input?
I know that my xlr input connector will go to one of the primaries, but which one (pins 1 and 5 or 2 and 6)?
I imagine the output of the secondaries (pins 3[+] and 4[-]) end up soldered to my edge connectors through pins 4(-) and 5(+). Is this correct?

Finally, any help and dirrection in incorporating phantom power, phase reverse switch and pad on these would be greatly apreciated.

 

[peter purpose]

Wotcha fur,
>that slot was replaced with R1. I did the same thing. Is this OK?!?
Yep, but could be left out as it was originally for a Jensen transformer.
>the resistor is first and the cap is second. Does the order of these matter?
Not entirely sure, but do it as in the schematic.
>unclear regarding my transformer connections

I think.... join 1&2 together and 5&6 together so that the primaries are in parallel.
1&2 will be your hot input, with 5&6 your cold.
>I imagine the output of the secondaries (pins 3[+] and 4[-]) end up soldered to my edge connectors through pins 4(-) and 5(+). Is this correct?
Yep. On the card I also join pins 3 and 4 together to ground.
>any help and dirrection in incorporating phantom power, phase reverse switch and pad
How are your pcb making skills?

peter

 

[peter purpose]

Oops...got the tripple 6..... not any more.

 

[furmound]

Thanks so much Peter.

So are you saying that I should remove R1 altogether since the modification on your cards are specific to a jensen transformer that i'm not using?

I'm working on combining the terminals on the transformers now.

My pcb making skills are alright. I had really bad results at first but I'm a little better at it now. Could I just use the pre drilled bread board stuff for phase/phantom/pad?

 

[soundguy]

If you want your gain pot to act as a full fader, try removing r3. With 47K in there, your gain pot will likely still pass signal when counterclockwise.

dave

 

[peter purpose]

Hi fur,

>So are you saying?

Yes. the 100K resistor was there for a Jensen 110K-HPC. The 312 schematic has only R4 5K1 & C5 220pF terminating the 2622.

>Could I just use the pre drilled bread board stuff for phase/phantom/pad?

Sure, but you could also just use a couple of DPDT and an SPST for the switching. Hang a couple of 6K81 reistors off the input XLRs and feed em +48V. No PCB. The wiring will have to be given some thought, but there are pcbs available to give you a far more elegant solution, with relay switching avoiding running mic level signals all over the box.

Dave is also correct in that with the gain pot fully counterclockwise, you'll still pass signal. With the Jensens in there and with 47K revlog / R3 47K, I had a gain range of around 30-60dB. With a 20dB pad in front, that becomes 10-40dB.

peter

 

[furmound]

Thanks for the info gents. I imagine the pots I'm using will be more "accurate" for stereo applications if the range is smaller, so I'll probably keep the 47K in there. Does that make any sense?

I will eventually add Bo Hansén's DI input mod and stepped gain controlls like the ones pictured in Peter's pre's anyway.

I saw some of the premade switching systems sold by some companies but I simply cannot afford those right now, especially since I'm making 8 channels. I'll have to figure out a way of incorporating these features in the future I suppose. I'm mainly concerned with pad and phase switches since I intend to use this box a lot with drums. Those features should be easier to incorporate right? And I can add phantom later.

Can I use Bo Hansén's schematic for the phantom power supply to make an external one for eight channels? Or is there some sonic reason why that's a bad idea?

[/img]

 

[soundguy]

you positively will need a pad to use these with a drum kit.

unless you are working for nasa, dialing in stereo is easy, dont make it a big deal that its not. One thing most people totally and completely overlook is that two microphones on a source does not automatically make it "stereo". Where level matching precision becomes necessary is with TRUE stereo in XY or ORTF or which ever mode you use, but to just set up mics on a source you dont have to be nearly as critical, just turn the knob until the meter shows the right level and you are done. Depending upon which pot you use, even if you dont want a full fader you might spend some time adjusting the value of that 47K resistor to get the swing you want out of the gain pot. You may get lucky and hit upon something you like immediately, but if you require more range, that would be how you could adjust it.

you can wire point to point hanging off switches or use a small perf board for the pad and phase, both are simple easy peasy to do point to point or on perf.

dave

 

[furmound]

Thanks. I understand your point completely. I was referring to critical stereo applications which I guess is silly anyway considering how inaccurate these pots are. I'll try the 47k's and if I don't like the swing I'll ask for more guidance.

Regarding pad and phase:

Are there schematics I can refer to?

I know these functions should be very simple to build. I mean, pad is just more resistance right? And phase is just a switch that reverses pins 2 and 3 from the xlr right?