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Old 23rd February 2011, 09:36 AM   #1
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Question Using a home preamp with a pro audio power amp

Hi Guys,

Here is the deal , how does one manage to use say a good unbalanced home preamp with a pro audio (cheap watts) power amp? , for example a NAD pre with a Crown Pro Power.

A couple of the issues are of-course running unbalanced to balanced ins rca to xlrs.But how does one get over the difference in output voltage of the pre and the input senitivity of the pro power amp,which would be at +4dbu as compared to the -10Dbv of the home pre.

Would one use a balanced to unbalanced line level shifter like a Rane balance buddy or a Ebtech LLS which are both passive (use transformers) or an active system like the ART clean box pro (uses op amps) .Would the active system not raise the noise floor of the system?

If we do not use any of the above , the only other option is to run the pro power amp on max open gain , here we risk , apart from frying whatever is connected , in-case a wire is loose or something , raising the noise floor of the system as well.

Are there any pro amps that accept -10dbv rca or xlr ? A few do have input sensitivity control , but most dont. So what would you do?
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Old 23rd February 2011, 10:21 AM   #2
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Originally Posted by SoundLogic View Post
So what would you do?
Build/modify the preamp to produce the required output. If it's op-amp based, you could even add a DRV134 for balanced outputs very simply.
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Old 23rd February 2011, 12:32 PM   #3
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I'm running my op amp Herald disco mixer into a Peavey CS800s amp. The amp has both unbalanced 1/4 phone plug and balanced xlr inputs. I've tried it both ways, in an unsucessful bid to remove some hum by using the xlr and differential op amp input of the amp. (Not the hum cause). If you need more gain out of your preamp, increase the feedback resistor of the output amp one size. If you need more voltage, increase the power supply voltages. I'm using +-7.9VDC, but +-3.3 VDC worked fine as long as the input to the op amps did not exceed 1 VAC. (my FM radio hisses less when putting out 4 VAC). When I hooked up the op amp outputs as balanced, I put RCA ring to minus and RCA tip to plus. The Peavey doesn't ground reference the XLR input. I got away from XLR because the SPC brand XLR connectors are such **** the insert melts when you solder it, and the case is so close and so loose it knocks the solder joints loose or shorts them when you pull it out. Use some other brand XLR. If your amp has the Nutrix combination input connector, the 1/4 phone is in the middle of the XLR input (was a shock to me when I finally got a user's manual for the amp).If your op amp doesn't have enough slew rate to sound good at the higher voltage, upgrade to ST33078 (if 8 pin DIP) or NJM4560 (SIP) which have 5.5v/usec. Using fast amps requires a disc cap (.01 to .1uf) between + & - Power suppies within an inch of the amp, and a 20 pf disc across the feedback resistors, to prevent radio oscillations. Upgrading from 4558 amps to 33078 amps cut the hiss at idle to lower than the fan noise of the factory across the street. (150') I don't know what technology the feedback resistors are, but they are cheap. (Metal film feedback resistors are the quietest).For the full details on the hiss & hum wars on my disco mixer, and the upgrades I made, see Improving a "Disco mixer" to mid-fi performance
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Last edited by indianajo; 23rd February 2011 at 12:57 PM.
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Old 23rd February 2011, 01:45 PM   #4
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if you want an undistorted (or as near as you can afford) output of +4dBu then you cannot use any conventional opamps which are limited to 44Vdc supply rails. And as a result none of the 36Vdc and 30Vdc opamps can do the job either.
+4dBu with 10dB of headroom (max distorted, but unclipped = +14dBu) requires a maximum output signal of ~5Vpk. This can be got from opamps if they have sufficient current to drive the cable loadings.
But much of the pro gear has outputs up around +22dBu to +24dBu.
+22dBu requires an unclipped 13.8Vpk to pass.
36Vdc & 44Vdc Opamps can supply that voltage, but only if the load resistance is high and the load capacitance is low. Not necessarily pro operational characteristics.

You need a high current buffer and opamp for a composite amplifier or you need to go discrete.

Or settle for much lower clipping to signal overhead and short cables to few amplifiers.
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Old 24th February 2011, 11:34 AM   #5
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Pro mixers that they feed pro power amps with sure do not have 36V op amps in them. A lot of them use 4558's. Rather than go to all that expense of exotic power supply and op amps, I would change the feedback resistor on the input op amp in the power amp to get more gain. Most crowns have an input PCB that comes out quite easily and uses conventional op amps. My peavey uses NJM4560's, that's how I knew they sound good in the mixer. (The 4560's weren't blown up, just the 1/8 W resistors in front of them).
I'm running a little short of gain on my RA88a mixer on the CD player input playing through the CS800s.I can't rattle the windows on organ record basslines, whereas my 40W Hammond organ amp (bass section) can. I can get nearly loud enough with LP input to the disco mixer, although the turntable suspension starts oscillating before the windows rattle. (it is between the speakers) So it is a gain in the disco mixer issue, or lack of gain from the line out of the CD player. As soon as I get another 68K resistor, I'm upgrading the final disco mixer op amp from 56K feedback resistor. The one surplus 68k resistor I have is metal film, I'll get another dozen in metal film and that should prevent additional hiss since the original resistors are probably carbon. Might upgrade the 1M feedback resistor on the LP mixer stage to metal film at the same time. Did the plate resistors in the PAS2 tube preamp to metal film, helped hiss out a bit.
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Last edited by indianajo; 24th February 2011 at 11:42 AM.
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Old 24th February 2011, 11:41 AM   #6
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do not have 36V op amps in them. A lot of them use 4558's.
what is the supply voltage range for the 4558?
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Old 24th February 2011, 11:50 AM   #7
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NJM4558, power supply max +-18V. The Peavey XR1200 mixer uses NJM4560 op amps on the master out, 3678 op amps in. Power supply is +-16V. 4560 has a lot less hiss than 4558 and more slew rate, my experiments with the disco mixer prove. But not +- 36V rails out.
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Last edited by indianajo; 24th February 2011 at 11:59 AM.
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Old 24th February 2011, 12:11 PM   #8
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Originally Posted by indianajo View Post
NJM4558, power supply max +-18V.
+-18Vdc is the same as 36Vdc
The maximum voltage that you can get from a 36Vdc opamp is ~17Vpk when feeding a very high load impedance.
Into cables and actual receiver resistances the peak voltage will more likely be ~15Vpk to 16Vpk.
But at these levels the current output of the opamp is the limiting factor. Opamps, in general, do not give out much current before very large distortions affect the signal. Expect 20 to 30mApk as the maximum output from a 36V opamp. This current has to feed the amplifier inputs and the capacitors fitted as RF attenuators and the capacitance of the cables hanging on the output of the opamp.
You need a line driver and that line driver must be designed for your installation.
A composite opamp and buffer can do this. See W. Jung
There is a current thread that addresses exactly this need.
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Old 24th February 2011, 02:58 PM   #9
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Default Pfffft - Too Much Theory, Just Try It Out....

Just make an unbalanced to balanced input cable, crank the input level controls and try it out.
Most standard hifi preamps run standard hifi output level at 11.00 o'clock position, giving extra gain at 5.000 o'clock which should be sufficient.
If you want lower level (and noise) just turn down the power amp input controls.

Eric.
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Old 24th February 2011, 03:04 PM   #10
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4560's will put out mininum of +-10V signal at 25 ma output. See datasheet on datasheetcatalog.com. That should drive a 100' snake + the amp input. Peavey mixers are used all the time on 100' snakes into their amps. Don't know if the piano makes that trip typically, but the guitars & basses sure do.
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