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cheap ways to increase input impedance?

dougi

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I have a minor, mainly conceptually problem I wouldn't mind help on. I am using my AVR (Yamaha RX-A2A, same as the RV-V6A measured here) front preouts to integrate with my 2 channel system. However, my measurements are even worse than Amir's for DAC performance at high output levels. I have traced this to the inability of the DAC to drive anything with an input impedance below 47kOhms well. My preamp (Lyngdorf DPA-1) and measurement device (RME ADI-2 Pro) both have 9-10kohm impedance.

Are there good, cheap products that may help increase the impedance the AVR sees? I can't seem to find a cheap, well performing buffer amp at all that is not DIY. Perhaps something in the pro field? My pre-amp immediately digitises analog inputs so even an ADC is an option. The pre-amp also has a balanced input so even a well performing unbalanced to balanced active converter (with high input impedance) would be an option as well.

Can anyone identify a suitable product?
 

DonH56

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dougi

dougi

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Thread Starter #3
Yeah that may be OK for the purpose. Cheers!

1612481980035.png
 

BDWoody

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dougi

dougi

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dougi

dougi

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Thread Starter #8
Agreed.

OP asked about an ADC...and already has (a great ) one in the RME ADI2 Pro...
Yeah but the input impedance is too low for the AVR. It wants >the 9k ohm of the RME. :(. -1dBFS 1kHz distortion of the AVR is ~-65dB into the RME (or the DPA-1) but reduces to at most 75dB into 47kohm. I tried this by using a tube headphone amp as a buffer. Distortion reduced but noise was unacceptable.
 
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#9
How about a JDS Labs Atom used as a preamp?
 
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dougi

dougi

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How about a JDS Labs Atom used as a preamp?
Input impedance is too low (10k). I like the idea of the iFi Itube2 (with all the silly stuff switched off). Up to 1 MegaOhm input impedance! Can't seem to actually buy one though and it is more expensive than an impulse purchase.
 
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#11
Change the two input loading resistors to 100k and your done. You don't need 1M ohm input z...that's gonna add to the noise and defeat what you say you're trying to accomplish. The real headscratcher is why the z-out of the Yamaha is so high. Have you measured it?
 
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dougi

dougi

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Thread Starter #12
Change the two input loading resistors to 100k and your done. You don't need 1M ohm input z...that's gonna add to the noise and defeat what you say you're trying to accomplish. The real headscratcher is why the z-out of the Yamaha is so high. Have you measured it?
I don't think it is the z-out, but I haven't measured it, just that it can't deliver even a smidgen more current than into a 47k load without compressing. The dac in it, PCM1502A, is supposed to be able to drive loads as low as 1k, but who knows how the internal amp stage is loading it. The DAC is also designed to drive loads directly, but perhaps not very well.
 

LTig

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#14
I have a minor, mainly conceptually problem I wouldn't mind help on. I am using my AVR (Yamaha RX-A2A, same as the RV-V6A measured here) front preouts to integrate with my 2 channel system. However, my measurements are even worse than Amir's for DAC performance at high output levels. I have traced this to the inability of the DAC to drive anything with an input impedance below 47kOhms well. My preamp (Lyngdorf DPA-1) and measurement device (RME ADI-2 Pro) both have 9-10kohm impedance.
@MC_RME wrote in a posting that it is not too difficult to increase the input impedance of the RME ADI-2 PRO to 100 kOhm, simply by replacing the resistors inside. I haven't had the courage to do it until now, but it’s on my list of TODOs.
 

MC_RME

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#15
The PCM1502 drives even lower than 1k. I can't believe the Yamaha's output is that high impedance. Your test with a tube buffer seems invalid, its high distortion might have changed the harmonics just in the way you saw. @raindance got the right tip, or just solder 47k in series into the connecting cable. The level that the ADI sees will obviously be lower, but that is no problem at all for the analysis.
 
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dougi

dougi

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Thread Starter #16
The PCM1502 drives even lower than 1k. I can't believe the Yamaha's output is that high impedance. Your test with a tube buffer seems invalid, its high distortion might have changed the harmonics just in the way you saw. @raindance got the right tip, or just solder 47k in series into the connecting cable. The level that the ADI sees will obviously be lower, but that is no problem at all for the analysis.
Yeah I have some RCA attenuators I can probably mod to just put a resistor in series. I will give it a try to confirm the issue, looks at the levels and take it from there. The tube buffer definitely reduced the THD distortion as did the RCA attenuators as is.
 
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dougi

dougi

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Thread Starter #17
I have modified the device I was using as a buffer. Musical Fidelity XCAN V3 tube/solid state head amp. I rewired it to redirect the phones output to the RCA loopthrough. Optimised the volume control to balance gain and noise. It definitely improves THD at high (-15dB+) AVR levels but a dB or two more noisy at normal (-30dB or so) AVR volume levels. Multitone not any real difference. It does roll of the bass a bit (-1.5dB at 20 Hz) but as bass management is filtering <40 Hz not a real problem.

direct.jpg xcan.jpg mtone straight.jpg mtone xcan.jpg

The question is do I need to keep it? The centre channel efficiency is ~87dBA/1W/1m. 8 ohms impedance. The AVR is setup for usual 0dBFS=105dBA reference level at the listening position (3.8m). The centre channel amp in the AVR clips at about 120-140W. This equates then to 96dBA at the LP or -8dBFS at clipping, where . So if I bother to really listen loud it's worth keeping it in to reduce overall THD. At normal levels probably not.
 

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