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Post by matt@IAA on Feb 27, 2018 12:57:16 GMT -6
I've seen literature that says the API 8200 used 2510 discrete op amps. Typically they use the 2510 in a balanced transformerless amplifier (BTA) setup. So, I don't know for sure but I suspect that they used a BTA for each balanced input. If you wanted to go to the standard 2520 footprint, you could definitely still fit 8 BTAs. Maybe even 16. With this you could do passive attenuation after onto the bus. This is probably the easiest and least expensive way. This signal flow would be: - Input jack - BTA input (2520 footprint) - Trim pot (passive) - Pan pot - L/R bus You could also simply set this up with each channel fixed at unity gain - a simple replacement of the DAW mix bus. This simplifies everything and lets the volume come from the digital side of things. Then you'd just need a pan pot. If you wanted to go bigger/ballsier, you could actually make it a mini console with proper line amps on each stage. Basically 325s, each with up to maybe 6-36 dB of gain. This adds the cost of eight transformers, but would probably sound bigger. You could leave the trim knob in there as a channel fader with 6 dB "in hand". That would be: - Input jack - Line input (2520+output transformer) - Gain (6-36 dB) - Trim - Pan pot - L/R bus The summing bus on an API uses two gain DOA and transformer stages for each side (left and right) - an active combining amplifier (ACA) and a second gainstage after the fader. You can see vanilla variants of all of these DOA configurations on this datasheet. So I guess the question is... what do you want it to do?
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Post by matt@IAA on Feb 27, 2018 18:46:21 GMT -6
So I was playing with the layout. I think it's gonna be pretty difficult to fit the full 325 style with actual line amps 8x in a 1RU box.
Some alternatives: 8 BTAs into the full summing bus (ACA + line amp stage) 6 full 325 line amps into the full summing bus 8 full 325 line amps into a unity gain ACA without the follower (possible disadvantage - more noise).
Or if you go to an external PSU, everything is fine and you can squeeze 'em all in there. Or go to a 14" deep 1RU.
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Post by matt@IAA on Feb 27, 2018 21:29:01 GMT -6
Just for fun...
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Post by mythundreamt on Feb 28, 2018 9:11:50 GMT -6
External PSU FTW!
The 2510 famously imparts no color, so the 2520 footprint is a must. So 12 op amps (including four for ACA + booster like you say) for an 8-channel or 20 for a 16-channel.
325 line cards instead greatly adds to cost and space though, what's the magnitude of the sonic difference? Also depends on which input trafo chosen, but assuming EA2622.
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Post by matt@IAA on Feb 28, 2018 12:00:12 GMT -6
Edit: I misunderstood your question. There's no 2622 or input transformer in a 325. It goes straight into the op amp. I thought you meant the 2623-4 PCB mount output transformer. I don't know, I've never tried to fit a 2623 in a 1RU. If you take the 1.416" height and add .060" for the pcb, plus a standoff, you're really close to running out of room inside. The way I drew it above is with 2503's laying down flat. That gives you a pretty wide range of possible options for transformers too, since its a very common footprint. With an external PSU and 12 full 325-style gain stages you get something like this (bussing on a separate PCB versus the one above where it's between the 8 BTAs). Pretty tight squeeze, but doable - especially if you go to a DB25 on the back panel for your 8 inputs. One nice thing about the 2503 is that the three secondary windings give you a ton of routing flexibility. You could take the first secondary at -2dB to the bus, and the other two in series for a +4 dB line out. At that point the 1RU becomes 8 535s PLUS a summing bus. So being clever, you have a DB25 for 8 inputs, another DB25 for 8 outputs, and then a L and R master out. The layout above also gives you the option of going with or without transformers on the same card for 325 or BTA inputs. However, since youre talking money, external PSU adds significant cost too. I think I could fit it all in a 14" deep 1RU - that's a pretty big case though!
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Post by mythundreamt on Feb 28, 2018 12:20:17 GMT -6
My mistake!! 312s have input 2622, 325 line cards don't have any input tranny. Musta been sleepy, sorry! So that would mean an opamp+transformer-laden mixer with direct outs, if I understand you correctly. 8 ins, say from a lunchbox filled with pres, 8 direct outs to A/D and then a summed mix to boot. Pretty incredible. So... what's it cost? Why does it have to fit in 1RU?
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Post by matt@IAA on Feb 28, 2018 12:44:25 GMT -6
Yes, exactly.
Each channel gets 3 pots (gain, vol, pan), mute switch, transformer and op amp, plus pcb and passive components. I'd guess maybe $75 or so each.
1RU and an external power supply and filtering - another $250. 2 DB25 jacks and 2 XLR or TRS outs for the L and R channels.
Figure $1200 bucks if you roll your own?
I guess it doesn't have to fit in a 1RU. But the limitation is the footprint, not the height.
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Post by matt@IAA on Mar 1, 2018 23:31:07 GMT -6
Is it cheating to use an IC op amp like an INA134 as the BTAs in the full 325 design? API uses the 2510 in this role for the 535.
Would anyone care?
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Post by mythundreamt on Mar 2, 2018 13:20:15 GMT -6
Probably. Losing the all-discrete badge is a big deal for some!
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Post by matt@IAA on Mar 2, 2018 14:13:18 GMT -6
Well, you can do the math there. An IN2134 is $4, and all you need is two caps to complete it. Call it $2.50 per channel. Each full 325 is about $75, each BTA is basically the same less transformer at $30 - $45.
So, three options:
8 BTAs into a stereo bus 8 IC BTAs into 8 line amps into a stereo bus 8 BTAs into 8 line amps into a stereo bus
I think if you went all discrete you could fit a second layer of DOAs in a 2RU. Phew! 20 op amps in a box!
I'm also thinking you put an additional pair of bus resistors as an aux input to the summing bus at -2 dB, and put a jack for unbalanced stereo bus out after t. That way you can do pre-summing ITB or daisy chain two of these together for mixing 16 tracks at once...
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Post by matt@IAA on Mar 5, 2018 17:29:26 GMT -6
Now I need to make some op amps.
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Post by mythundreamt on Mar 6, 2018 10:53:41 GMT -6
Impressive! You..you planning to sell this by any chance?
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Post by matt@IAA on Mar 6, 2018 11:31:55 GMT -6
If it works
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Post by rocinante on Mar 18, 2018 9:33:14 GMT -6
Its awesome. I built my neve ended mixer using a 325 variant and app992s as well as a few jlm hybrids. Needed 24v for the never circuitry. A clever dude from Turkey named Forthmonkey designed the channel boards and had them for sale for a good minute at GDIY.
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Post by Blackdawg on Mar 19, 2018 15:14:24 GMT -6
Now I need to make some op amps. Brad at LTLO sells op amps for only $19. They are very nice sounding as well.
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Post by matt@IAA on Mar 19, 2018 15:20:14 GMT -6
Where's the fun in that?
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Post by kilroyrock on Mar 30, 2018 7:05:42 GMT -6
Hey matt@IAA - I'm looking to build some pcbs - what program did you use for that? How over my head would I be if I tried? I'm sure you've got years of experience, but you make it look pretty easy..
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Post by matt@IAA on Mar 30, 2018 9:13:02 GMT -6
Eagle and AutoCAD for some stuff. EasyEDA for others. EasyEDA is pretty straightforward, but there is a learning curve.
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Post by matt@IAA on Mar 30, 2018 15:24:49 GMT -6
Update - I have one more homemade op amp to try out, then I’ll build start to build one out. But, for me next week is Holy Week so I probably won’t have a ton of time to work on this for a bit.
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Post by iamzeke on Apr 4, 2018 8:31:32 GMT -6
Are you planning to make this so it can integrate with existing 8200's via the Ribbon Cable? I'd be interested if so. Also, I wouldn't mind adding more 7600's to my rack if you plan on making something like that too. my current 7600 have heating issues (all 8 of them) but they sound great.
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Post by matt@IAA on Apr 5, 2018 7:01:57 GMT -6
iamzeke I hadn't planned to, but it's a super-dee-uper prototype at this point. I found a crappy manual scan but I can't read the pinout clearly. Right now this is just 8 balanced transformerless 2520 inputs across passive panning into a stereo bus, which then dumps into a virtual earth summer. What would you want it to receive from another 8200? And for the 7600, I actually have a prototype that's further along in that aspect, but it has only the line/DI/mic input and a single 500 slot, with all of the appropriate gainstaging (including a channel master fader with dB in hand).
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Post by iamzeke on Apr 10, 2018 10:11:48 GMT -6
Thanks for the reply matt@IAA, I have the manual for the 8200 at home. Is this the pin out you were looking for? As for the 7600 style that would be good as long as I can use the ribbon cable pin outs with that too.
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Post by matt@IAA on Apr 10, 2018 10:54:10 GMT -6
Yes, thanks. Does your manual have a block diagram in it?
To be honest I don't know if making it compatible with that sounds like all that much fun.
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Post by iamzeke on Apr 10, 2018 11:39:05 GMT -6
API_Audio_8200_Summing_Mixer.pdf (1.7 MB) Yes I believe it does. I think it may be on the case of the 8200 too. I could be confusing it with the API 7600 though.
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Post by matt@IAA on Apr 16, 2018 9:35:32 GMT -6
Got my power supply working, and my homebrew DOA going. (1/2, I was going to use my own version of a Quad Eight AM10 but I'm having some issues with that one...). Going to solder up the other 10 op amps and build the board out.
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