2k for XLR, 10k for RCA.
Frequency response will not affected by high output impedance devices unless it's capacitor coupled or transformer coupled.
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Topping Pre90 Review (preamplifier)
- Thread starter amirm
- Start date
Frequency response will not affected by high output impedance devices unless it's capacitor coupled or transformer coupled.
Actually can you shine a bit of technical detail of the choice? It seems rare to have xlr input impedance lower than rca
restorer-john
Grand Contributor
Just as I noted in my post.
As we discussed in another thread, it's simply passing the buck from one stage to another. A preamplifier (that's supposedly what this is) with an input impedance this low (~2K or less) is a faulty design. It's only configured this way, solely to get great numbers on paper. It has one single legacy RCA input at ~10K but that hasn't been tested in this review. Let's see a typical CD player source impedance of 200R-470R and a series ~10uF cap FR plot into this.
Preamplifiers are intended to bring multiple devices with variable output levels, impedances and capabilities together, to switch, buffer and normalize in order to drive the next stage. They are not used in isolation. That's what these AP numbers jockeys don't realize.
You would need 2 extra dual opamps to buffer the balanced input. This occupies board area and adds cost. This also adds a lot of noise if you select the disconnected balanced input. Low input impedance is also better for rejection to interference. No good in almost every way just for a little compatibility for capacitor coupled output devices which aren't considerately designed in the first place.
Increasing input impedance doesn't reduce performance.
restorer-john
Grand Contributor
A proper preamplifier does not load down, skew frequency response, produce distortion, or result in an inferior performance in the source component.
Your preamplifier's input impedance is ridiculously low and you know it.
Your preamplifier's input impedance is ridiculously low and you know it.
I agree totally, John. What they only care about are numbers on paper and they profit from "ignorance" of vast majority of users. Paper numbers are just different marketing strategy, however solely marketing strategy. I would love if there was a neutral field (to discuss), but it is probably impossible. And, there seems to be a direct thread to .... I better stay silent.
It would be interesting to see how couple of DAC sources, with a high output impedance, are measuring with this preamp, given the possible impedance mismatch. I wonder if there will be bass roll-off with some sources, especially with those having output caps.
restorer-john
Grand Contributor
I'm pretty sure 95%+ of dacs measured by Amir are fine with this.
restorer-john
Grand Contributor
John, you don't seem to realize that a preamplifier, by it's very nature, deals with a whole lot more than just a DAC which Amir may have measured.
That is indeed, the entire reason preamplifiers exist, and have done for 70 years or more. They are the engine-room, the grand central station if you will, for various components to come together and be treated in such a way that their strengths and weaknesses are addressed, in order to extract the very best performance from each to drive the next stage (the amplifier).
As I said, preamplifiers do not exist in isolation, and crazy good numbers mean absolutely nothing in the scheme of things without context and conditions. Your preamplifier will be torn apart in a proper HiFi review for the stupid low input impedance. You are an excellent designer no doubt, but as is typical with some engineers, they cannot see the wood for the trees.
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Foxenfurter
Active Member
Who is this targeted at? I am not convinced with the inputs and the input selector the use cases have been thought through. Adding the extra inputs on an extension box is a clever idea to keep the form factor and limit the expense for those who need it, but it compromises the design of the input selection meaning more button presses.
As others have said the inputs on the extension box are too biased towards balanced for most listeners and the ergonomics are too clunky for a pro.
I assume that you have to cycle through connections on the remote. Are there plans for a second add-on box with more single ended connections?
As others have said the inputs on the extension box are too biased towards balanced for most listeners and the ergonomics are too clunky for a pro.
I assume that you have to cycle through connections on the remote. Are there plans for a second add-on box with more single ended connections?
Phrangko
Member
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I've been searching for the elusive preamp after I sold my lame integrated/preamp. But preamps are f.....g expensive. The price of the Pre90 is half of what I've been evaluating. Amir's test is mostly ceremonial with regard to Topping. Still the Pre90 deserves its coronation. I'm no technical person and I owned a D70 and now the L30. I think and most likely, I'm getting one.
Does this mean that transformer isolators like Radial SB-6 are not advised? They have an output impedance of 600 ohms.
Except for analog inputs. The Topping D90 is closer to that for about half the price. RME better DAC overal for sure, though.
Nice to see such performance !
@amirm can you give the THD vs volume adjustment ? To show the effect of the internal gain switching. This measurement has been shown in another thread but it will be interesting to see it here
And thanks again for the good work !
EDIT : and the channel match if you have done the measurement
@amirm can you give the THD vs volume adjustment ? To show the effect of the internal gain switching. This measurement has been shown in another thread but it will be interesting to see it here
And thanks again for the good work !
EDIT : and the channel match if you have done the measurement
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Tatteredmidnight
Active Member
But how many DAC’s do you see people realistically hooking up to a pre? I have to agree with Jon on this, isn’t the whole point of a device like this to aggregate all your sources for use with a single output chain? I think I’m missing the envisioned use case for a pre with a large number of low impedance, balanced inputs without the ability to directly switch between them (sequential selection only).
p.s. A second output as mentioned by another user would add a huge amount of additional value for integrating a HPA and monitor controls to a single device, and I’d even add that a summed mono output for a sub on a secondary output would be icing.
I can only show an example of transformer with output impedance of 75 ohms. If there is a capable output that drives the transformer, then transformer loading will most probably flatten the frequency response a bit and make low frequency distortion a bit higher.
Frequency response with trafo loaded by 51k (green) and 2k (blue)
Distortion at trafo output loaded by 51k and 2k
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