WolfX700 Measurements of Topping Pre90/Ext90 Preamplifier/ Extender

[JohnYang1997]

@JohnYang1997
I was waiting for this Pre impatiently, but now some people say that the Pre90 is not universal due to the to low input impedance. It will be correctly using this Pre90 with configuration like on attached picture?

It will be fine if the dac is single ended output (RCA).

 

[gvl]

There's no industry standard first of all.
Secondly if it measures fine it's impossible to sound bad. WTF is 'kill the good sound"?

The analog output of the analyzer is not a good representation of all devices. Add a 200 Ohm resistor and a 10mF capacitor in series and post the resulting frequency response.

 

[The Messiah]

I would have liked to see more coloured flashing lights. Like green, red and orange flashing. A few extra buttons just for show would be cool as well. Helps impress my girlfriends and apostles .

 

[Harmonie]

I would have liked to see more coloured flashing lights. Like green, red and orange flashing. A few extra buttons just for show would be cool as well. Helps impress my girlfriends and apostles .

You are not on the right thread:

THIS is what YOU need :

https://www.audiosciencereview.com/forum/index.php?threads/chord-huei-phono-preamp-review.17355/

 

[MusicNBeer]

2K input impedance? Wow that is low. That does limit possible source gear to robust output stage only.

 

[ririt]

2K input impedance? Wow that is low. That does limit possible source gear to robust output stage only.

I would appreciate if someone can disseminate the consequence of this low impedance on the devices to associate with this preamp. I am not an expert in such matters.
thanks in advance

 

[Deleted member 18309]

There's a lot of discussion about the low input impedance. I'm not an expert, but I understand it's not an issue if the source has low output impedance right? Looking at the fact that it's got A90 footprint and there are only 2 inputs (1 balanced, one single-ended), I think maybe the target audience is those who use a simple system with one (or at most two) digital sources and/or sources with low output impedance? For example, I imagine I wouldn't have issues using Pre90 with E30 DAC (100 ohm output impedance if I recall correctly) or Schiit Modius (75 ohm output impedance). I don't know about the average output impedance of most DACs, but I imagine they're usually not that high?

Just to be clear, I don't have a lot of knowledge and it is also not the intention to "take sides" here - I'm just trying to brainstorm why exactly the choice to not design a preamp with higher input impedance was made. Sure, maybe it gives great measurements. But I think the consideration of who Topping is trying to attract plays a part here too. If people with other kinds of sources with high output impedance wants a preamp, I think it may have been taken into account but ultimately decided during the design process that Pre90 is not designed for such people.

Please feel free to share if this makes sense.

 

[tmtomh]

I'd just like to add my voice as another non-expert for increased clarity on the input impedance issue. Specifically, is there any way for us to know if, say, a "typical" (if such a thing exists) modern source device like a DAC or CD/Blu-Ray/Universal disc player that provides 2V/4V output via unbalanced/balanced outputs, would have a problem with the PRE90's input impedance?

In other words what kind of "older gear" would have an issue? And what does it mean exactly that a "robust output stage" on the source component is required - what exactly is meant by robust? And are there any ways we can get a sense of this if the source component's published specs do not include its output impedance?

 

[ririt]

To further try to understand...I was thinking to connect both a Dac and my Denon AVR4700h on this type of preamp. Amir determined the output impedance of the Denon to be about 1.2kohm. Does that fit with the input impedance of the pre90?

 

[gvl]

I don't think there's many of those who want to switch between several low output impedance modern DACs using a preamp. The more realistic use case is someone with a turntable, tape deck, and a digital source. The first two are often older gear with a DC cap on the output . Caps have higher impedance on low frequencies which can lead to signal level droop in the low end. If the source is DC coupled and it's output impedance is low and constant across audible frequency range this pre should work fine.

 

[win]

I don't think there's many of those who want to switch between several low output impedance modern DACs using a preamp. The more realistic use case is someone with a turntable, tape deck, and a digital source. The first two are often older gear with a DC cap on the output . Caps have higher impedance on low frequencies which can lead to signal level droop in the low end. If the source is DC coupled and it's output impedance is low and constant across audible frequency range this pre should work fine.

If that's the case, is it fair to say that those with such older equipment are probably not after the measurement game that this preamp seems to appeal to?

 

[gvl]

If that's the case, is it fair to say that those with such older equipment are probably not after the measurement game that this preamp seems to appeal to?

If older gear is all they have then probably not. I was hoping for a pre that could help to integrate old and new gear with no side effects however.

 

[EdW]

I would appreciate if someone can disseminate the consequence of this low impedance on the devices to associate with this preamp. I am not an expert in such matters.
thanks in advance

Why 2k?
A 2k input impedance perhaps indicates the use of an opamp in the inverting mode, rather than voltage follower mode. 2k is chosen as the input resistor to the virtual earth input of the opamp in order to reduce noise. This configuration which does give a low input impedance however does not rely on the common mode rejection ratio (CMRR) of the differential I/P of the opamp which is the case in voltage follower mode where the differential input pair follow the input voltage resulting in sensitivity to the CMRR parameter and hence a marginally higher distortion.

The consequences for the driving source?
1. If fed from a high quality opamp (capable of sink/sourcing several mA) in the signal source with a low value series resistor between the opamp and the O/P terminal of the signal source (as in the D90 for example), then it works perfectly.
2. If the series resistance is higher then signal level is lost: so if for the sake of argument the O/P Z is 2k you lose 6dB of gain
(and add inconsequentially to the noise)
3. With only 10dB of gain and the above signal loss you may lose out on the ability to fully drive your power amp if it’s not sensitive. So check the drive levels of the signal source - some were designed in the past to work with line input sensitivities of ~250mV on an integrated amp
4. To drive 2v rms sine wave into 2k load then your source needs to be able sink/source 1.41mA with low distortion so sources which don’t have a feedback opamp with a complementary O/P stage will suffer severe distortion. Such a stage might be an emitter follower O/P which was used in the past when a high impedance load was anticipated

 

[ririt]

Why 2k?
A 2k input impedance perhaps indicates the use of an opamp in the inverting mode, rather than voltage follower mode. 2k is chosen as the input resistor to the virtual earth input of the opamp in order to reduce noise. This configuration which does give a low input impedance however does not rely on the common mode rejection ratio (CMRR) of the differential I/P of the opamp which is the case in voltage follower mode where the differential input pair follow the input voltage resulting in sensitivity to the CMRR parameter and hence a marginally higher distortion.

The consequences for the driving source?
1. If fed from a high quality opamp (capable of sink/sourcing several mA) in the signal source with a low value series resistor between the opamp and the O/P terminal of the signal source (as in the D90 for example), then it works perfectly.
2. If the series resistance is higher then signal level is lost: so if for the sake of argument the O/P Z is 2k you lose 6dB of gain
(and add inconsequentially to the noise)
3. With only 10dB of gain and the above signal loss you may lose out on the ability to fully drive your power amp if it’s not sensitive. So check the drive levels of the signal source - some were designed in the past to work with line input sensitivities of ~250mV on an integrated amp
4. To drive 2v rms sine wave into 2k load then your source needs to be able sink/source 1.41mA with low distortion so sources which don’t have a feedback opamp with a complementary O/P stage will suffer severe distortion. Such a stage might be an emitter follower O/P which was used in the past when a high impedance load was anticipated

Sorry @EdW this much too technical for me! If you can simplify your explanations and give me an idea about the kind of specs I need to look at for the sources to be connected with this preamp, it will be great!

 

[MusicNBeer]

Low input impedance means the source has to supply more current to keep the voltage linear. If the source component has robust op-amps on the output that can supply the required current, all is fine. If not, the source component will increase distortion into the preamp. Same idea with power amps increasing distortion into 4 Ohm, 2Ohm, etc.

Low input impedance on the preamp also requires the source output impedance to be very constant with frequency, otherwise the frequency response will be affected. This is what the earlier post was referring to with regards to low frequency droop.

 

[Pdxwayne]

Low input impedance means the source has to supply more current to keep the voltage linear. If the source component has robust op-amps on the output that can supply the required current, all is fine. If not, the source component will increase distortion into the preamp. Same idea with power amps increasing distortion into 4 Ohm, 2Ohm, etc.

Low input impedance on the preamp also requires the source output impedance to be very constant with frequency, otherwise the frequency response will be affected. This is what the earlier post was referring to with regards to low frequency droop.

Curious, when using DAC with weak op-amp, would the distortion cause excess highs and recess low? Thanks!

 

[EdW]

Sorry @EdW this much too technical for me! If you can simplify your explanations and give me an idea about the kind of specs I need to look at for the sources to be connected with this preamp, it will be great!

So let’s take a practical example. The sort of power amp you might be using this with would be the Benchmark AHB2. This needs 2v rms drive for its nominal output. You choose to set the PRE90 to +10dB gain in order to minimise the drive requirements for the signal source which means your signal source has to be capable of driving 632mV rms into the 2k load of the PRE90. So if the output impedance of your source is much less than 2k (say 100 ohms) then your source must be capable of 632mV absolute minimum. If the impedance is higher, say 2k, then you’d need 1264mV of output. But be careful: a high output impedance from your signal source suggests that it was never intended to drive a low impedance and probably will distort if loaded that way. A modern signal source will probably incorporate an opamp output stage and will do the job OK (boutique phono preamps excepted). Older gear probably won’t make the mark. The manufacturer of the signal source is very unlikely to give the full spec on the output capability of his product - because he has never measured it!

 

[gvl]

The manufacturer of the signal source is very unlikely to give the full spec on the output capability of his product - because he has never measured it!

This is key. In other words pairing with low impedance input is crapshoot.

 

[EdW]

Curious, when using DAC with weak op-amp, would the distortion cause excess highs and recess low? Thanks!

Nearly all opamps would do the job just fine and would be capable of sinking/sourcing several mA without significant increase in distortion above the lightly loaded case. If the O/P is particularly weak (say a low power battery DAC) just set the PRE90 to +10dB gain then you’ll need less drive capability.

 

[Harmonie]

Low input impedance means the source has to supply more current to keep the voltage linear. If the source component has robust op-amps on the output that can supply the required current, all is fine. If not, the source component will increase distortion into the preamp. Same idea with power amps increasing distortion into 4 Ohm, 2Ohm, etc.

Low input impedance on the preamp also requires the source output impedance to be very constant with frequency, otherwise the frequency response will be affected. This is what the earlier post was referring to with regards to low frequency droop.

Seems quite an issue:
I might be wrong:
Is this impedance/current relation measured each time we have a new review here on sources, preamps aso ?
Or:
The fact that we just measure distortion, is the answer to everything ?