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Amplification stages vs. preamps vs. the 'Gain Cell'
Amplification stages vs. preamps vs. the 'Gain Cell'
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Old 16th March 2019, 04:35 PM   #11
indianajo is online now indianajo  United States
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If you need to drive 6 amps with different frequency ranges, you have frequency filtering in your hub. 6 different cables is an invitation to rf/hum/pop interference. Each twisted pair cable introduces about 20 pf per foot which filters highs off high impedance sources. I would say you need 6 different op amps (maybe 2 to 4 per IC) to drive the 6 cables. Each has separate gain pot on the panel, and several of them are not full frequency and need a pre-filter which is most economically done with op amp stages these days. This would be custom, even full feature mixers have typically 2 stereo output channels, main & monitor. You could use the headphone out as a third ajustable level amp drive, but the frequency control of headphone in a commercial mixer is limited.
Your three high level sources have XLR cables, which can pick up RF/hum/pops, so some filtering at the input is indicated. I use 33 pf ceramic to case ground. The balanced input lends itself to op amp in the differential mode to discriminate common mode noise, although you might be able to get away with passive pot attenuation. However, mixing them to the power amp bus requires 1000 ohm minimum mixer resistors to keep from blowing up the drivers in the sources, and that attenuation is usually followed by an op amp to boost the signals back up to 2 vac. 10000 ohm resistors doing the mixing are more usual, there are reasons covered in westsound's long discussions of mixers. No point IMHO mixing both top and bottom of the xlr IMHO. You could use transformers to input the differential signals from the XLR and make them ground referenced, but these are more expensive than op amps, running about $90 each for the good ones (jensen, sennhauser). Instead of mixing the various inputs you could put a rotary stereo switch which is expensive these days, or various LDR attenuators controlled by toggle or slide switches. Farnell/digikey/mouser don't carry stereo rotary switches anymore, but I think dynaco upgrade hubs like dynacodoctor do. the dynaco rotary switches shorted the unused inputs though to prevent pops, which is not compatible with active drive sources. But if you don't follow the LDR attenuators by 1000 ohm resistors & a mixer op amp, if you turn two on by accident, you can blow up both source output (op)amps. Better in that case just to switch cables on a single input xlr jack.
BTW a steel box is required to keep out the RF lightning pops etc. If you put the power transformer in the device, a second steel box or bulkhead is required to keep the hum out of the internal amps. Particularly if you have a 50x gain amp for mm LP. Moving coil LP is more like 80 x gain. I proved this point when I turned a hummy hissy $15 disco mixer into my entertainment hub Improving a disco mixer to mid fi is the name of the thread where I detailed that exploration. & I am at about 75 db s/n not 110 as sgrossklass and PRR talk about.
I safety ground the hub box, and not the power amp to avoid the ground loops case ground to hub case through both the xlr shield and the building wiring. Three of my power amps are single ended and have RCA jacks grounded to case with a 2 prong AC cord. The ungrounded power amps live behind the organ between the speakers and are not a safety hazard because no babies crawl around my floors. Modern power amps with an op amp on the input to differentially reject the common mode hum can have the case grounded. Op amp ICs became useable for low hiss low hum hifi about 1985. Even the ~1980 Peavey cs800 rev A, B, & C that had differential input op amps were too hissy. CS800x & CS800s are dead quiet.
I recommend low hiss op amps like MC33078, njm2068, NE5532. Maybe LM4562 on output, although it is a bit high powered and fast for audio. Peavey gets away with NJM4580 for 100' XLR cables from mixer to stage amps.
Have fun building your system.
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Last edited by indianajo; 16th March 2019 at 04:50 PM.
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Old 16th March 2019, 04:53 PM   #12
scottjoplin is offline scottjoplin  Wales
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Quote:
Originally Posted by Vortex View Post
Would you please have a look at this short vid ? (5 mins). YouTube
"20 years worrying about pots" I bet he'd have a heart attack if he ever saw a mixing desk
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Old 16th March 2019, 06:58 PM   #13
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He has just installed one.. YouTube
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Old 16th March 2019, 07:02 PM   #14
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Quote:
Originally Posted by indianajo View Post
Have fun building your system.
Uh that's heavy stuff, still digesting. Many-many thanks for these tips.

For OpAmp selection: one of my friends, an op-amp specialist gave me these hints:
- Opamps have low distortion in Class A mode
- Recommended types are OPA2209 or OPA2140
- PSU: strictly use K-multiplier type, nothing else
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Last edited by Vortex; 16th March 2019 at 07:15 PM.
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Old 16th March 2019, 10:56 PM   #15
indianajo is online now indianajo  United States
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I use speakers, and mine have 2nd harmonic distortion down 20 db at all frequencies 54 hz-14khz at 1 W. That is the volume I listen at, and I think guys that claim they can hear .01% HD are full of it. On headphones, maybe, if a guy has never fired a gun or a firecracker without earplugs,never been too close to an airplane or a poorly muffled motor, never used a power saw without earplugs, never been near the stage at a rock/techno/house/ concert without protection. Most men at age 25 have no hearing above 8 khz: mine was tested to 14 khz 11 years ago.
I can hear 1 % hd, my ST70 amp is about that. Fuzzy sound. My "world's worst amp" the ST-120 with djoffe bias mod, and my no-capacitors in the sound path .02% HD CS800x sound exactly the same on these speakers at 1/4 to 1 watt my normal soft passage level.
There is nothing wrong with the OPA2140 except the $6.50 each price for the TSOP package that takes a $2 adapter to be used by ordinary mortals without a wave solder machine. I think my Peavey mixer with 4560 & 4580 op amps sounds just fine, as does my RA-88a after I changed the 4558 to 33078 and purified the power supply hum and straightened out the grounding. 33078 cost me a whole $.38 each and the legs went nicely into the phosphor bronze sockets I installed so I can try some op amp more exotic if I find more purity is in order. Not likely this decade. My next projects are a 30W power amp for my summer camp so I can hear the FM radio while outside & working, and a $38 PV8 mixer for the church so I can mix my wimpy voice from a face mike into the Allen organ speaker system, while I play piano and sing.
My power supply hum problems were solved with a $1 surplus race car 18 v wall transformer, a toroid choke at the inlet to the mixer, 3 470 to 1000 uf caps with intermediate resistors as a pi filter, and two 1n5344 zeners to regulate the +-7.8 voltage into 2 caps for the op amp rails. The biggest fault I see in the RA-88a is not those, it is the approximate RIAA curve that could use much more expensive caps to be more accurate. If they would fit.
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Last edited by indianajo; 16th March 2019 at 11:15 PM.
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Old 17th March 2019, 08:01 AM   #16
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To #11: not sure if I could place all this in context for myself.. let's try it: I don't want to mix my sources. Just switch between them with an input selector (analog).

Mostly the computer would be used, almost 99% of the time.
DAC's XLR Output :
4.5Vrms Fixed at 0 dBFS DAC MODE
0-4.5Vrms Adjustable at 0 dBFS PRE MODE (using it as a digital preamp)

If I connect an analog electronic crossover like this one after the DAC (with values adjusted), that should be enough I think and I can connect the XO's outputs directly with the power amps.

I'm just thinking about where to adjust gains for the channels due to somewhat different speaker sensitivities. Either on the amps themselves (Class D: attenuation, mid/tweeter amps: either attenuation or gain cell as input stage).

A master volume "pot" is only needed for the non-DAC source, else I can adjust my master volume from the DAC itself, using it in PRE mode.
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Old 17th March 2019, 08:59 AM   #17
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When using a miniDSP with built-in DAC, it's the same I think. It has enough drive on its XLR outs to feed the 6 amps.
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Old 17th March 2019, 12:34 PM   #18
indianajo is online now indianajo  United States
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That electronic crossover from sound.au.com seems a marvel of op amp technology. Not only splits your sound 3 ways by frequency, it contains buffer amps to adjust the levels to the 3 individual amps. I can't see on the picture or schematic what IC they are using.
If you have two sources you can use a dpdt toggle switch, but if you have 3 you need a rotary or a logic selector for 1 of 3 plus relays or LDRs. I did find a 3 position rotary that would work but no telling if the contacts are gold: https://www.newark.com/electroswitch...15v/dp/06M4673
There is also a 5 position 4 pole version to allow for future expansion for LP + CD sources: https://www.newark.com/electroswitch...15v/dp/06M4668
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Last edited by indianajo; 17th March 2019 at 12:38 PM.
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Old 17th March 2019, 01:16 PM   #19
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if you have all the high-resolution DSP, DAC following each other, you can control volume from there in the digital domain and skip the volume control.

The easiest route however is to have a pre-amp with selector connected to a power amp. You feed it with DAC or whatever source, like phono, ipod, and send the pre to the power amp.

For the woofer stage you take that power amp output, and feed it to an integrated amplifier with tone controls to filter the high frequency.

This enables you to have smaller cabinets , by using dedicated woofers.

Mid range and tweeter are better used with a XO and driven by separate amps with same pre-signal.

So in the ideal world:

Sources:
AUX 1(iphone)
2(computer)
3 (TV)
PHONO
DAC server

sources -----) pre-amp (volume control + gain +buffer)

Pre-amp has an output splitter.

Pre-signals to a)----- power amp 1 (same gain as power amp 2)---- Tweeter XO
b)------ power amp 2 ----- mid/woof XO

Signal from either power amp 1 and 2 -------- attenuator ---- filter ---- integrated high power subwoofer amplifiers (can be either stereo with 2 subs or /mono/2 channels to mono)
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Old 17th March 2019, 05:16 PM   #20
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