WolfX700 Measurements of Topping Pre90/Ext90 Preamplifier/ Extender

[EdW]

Look at post #83 and than #84. Do you think that JohnYoung could lie to me?

You have misread what I have said:
In the case you discuss with John we can analyse as follows - assuming the drive is from an opamp in your DAC (DACs usually have opamp outputs unlike older signal sources) then the signal will be attenuated by 3.5dB because of the 1k feed resistor and the low frequency cut off would be about 11Hz with the 4.7uF coupling capacitor - so OK

 

[seashell]

Why 2k?
A 2k input impedance perhaps indicates the use of an opamp in the inverting mode, rather than voltage follower mode.

[sarcasm]Inverting? Should we start the reverse absolute polarity discussion now? Was it designed to be paired with the E30?[/sarcasm]

 

[gvl]

nd the low frequency cut off would be about 11Hz with the 4.7uF coupling capacitor

How is this calculated? 4.7uF will add about 1k impedance at 35Hz, no?

 

[EdW]

How is this calculated? 4.7uF will add about 1k impedance at 35Hz, no?

The 4.7k is feeding 1k in series with the 2k I/P impedance of the preamp giving a 3k effective load so a time constant set by 3k and 4.7uF

 

[MusicNBeer]

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

It causes compression in the signal, meaning amplitude peaks get "soft clipped" for lack of a better term. This is harmonic distortion which would be easily measurable at the preamp output if present.

Many source components will say "minimum load impedance" of 10K or so to avoid this and the frequency response issue.

One area that low load impedance helps is with noise pickup in the cables. With balanced cables, this is pretty moot though.

The 2K Ohm input impedance value is interesting because it's exactly the input impedance of the Adcom GFP-750 in passive mode. I have one and measured it. I remember back when that preamp was popular, people were always saying it sounded like crap in passive mode. My theory is this exact issue.

 

[Max]

So, based on that measurement from Wolf, this has granular volume adjustments down to -24db, but lower than that are jumps of -5db and -10db. Am I interpreting that right?

No. You can see the whole volume adjustement in the first page and the granularity is "continuous" from the beginning to the end. The second measurement has been made by Wolf to answer quickly my question about THD vs volume adjustment. This measurement requires to adjust manually the volume and to measure at each point ... so, Wolf has well analyzed the first part where the gain switching occurs then has just used big steps down to finish the second part.

 

[Neale]

I was hoping Topping was going to release a pre amp that includes a DAC. Why can't someone, anyone make a single box with a DAC and a couple analog inputs. This is all anyone who hangs out at ASR needs .. we know good sound come from a digital source, but we all have a couple of analog devices we still use ...

 

[Vini darko]

I was hoping Topping was going to release a pre amp that includes a DAC. Why can't someone, anyone make a single box with a DAC and a couple analog inputs. This is all anyone who hangs out at ASR needs .. we know good sound come from a digital source, but we all have a couple of analog devices we still use ...

Aylic s50 pro streamer @£170 does that. Only single ended and not sota but the price reflects that.

 

[MRC01]

Relay of course.

SSR or EMR? Is the volume silent as you move it, or can you hear the relays clicking? What is the expected relay lifespan?

 

[Music1969]

I've seen comments that high output impedance with DACs is a way to cheat for a higher SINAD.

Does similar apply to this Pre, with low input impedance? Does that aid SINAD measurement ?

 

[Vini darko]

I've seen comments that high output impedance with DACs is a way to cheat for a higher SINAD.

Does similar apply to this Pre, with low input impedance? Does that aid SINAD measurement ?

It can mean being able to run less gain and have a lower noise floor as a result.

 

[VeerK]

The 2k impedance is interesting, assuming Topping intends for the Pre90 to be used with their eventual power amps. Definitely sitting on the sidelines for now

 

[EdW]

I've seen comments that high output impedance with DACs is a way to cheat for a higher SINAD.

Does similar apply to this Pre, with low input impedance? Does that aid SINAD measurement ?

Generally low value resistors generate less noise
Noise voltage = SQRT(4KTRB)
where SQRT stands for square root, K is Boltzmann’s constant, T is temperature in degrees Kelvin, R is the resistor value Ohms, and B is the bandwidth of interest (Hz)
so a low source resistance from the DAC is good here

The noise trade offs in the preamplifier would depend on the architecture of preamp. The lowest distortion virtual earth feedback design will give lowest noise when the input resistance is low. A voltage follower architecture would have slightly higher distortion (could still be around -120dB linearity) but without the need to load the signal source.

 

[gvl]

The L30's pot is wired directly to input, perhaps this pre's design is similar. Is this the "virtual earth feedback design"?

 

[sq225917]

Re the battery shipping. They can leave the battery in the plastic cardboard strip it was supplied in or put it in the remote with an electrically insulating pull tab fitted to stop it making contact on both electrodes. I assume its a sliding door type remote where the electrodes touch the battery on the front and side, so option one is the only choice for them.

One battery per box, perfectly legal for intl Air shipping.

 

[restorer-john]

There's no industry standard first of all.

Sorry on this one, you're mistaken. 2K is ridiculous and you know it, John. You've gone after one metric at the expense of source compatibility and performance. It's a mistake to pass the buck to the source components. A preamplifier is designed to take varying source impedances and levels and buffer them without messing up the performance of the attached components.

47-50K is the defacto input impedance for line sources into a typical preamplifier/integrated line input. It's also the defacto input impedance of an MM RIAA stage and has been this way for many, many decades. Go peruse a few hundred service manuals for amplifiers and preamplifiers and tell me what the "standard" is. I can tell you, it's closer to 50K than 2K.

20-25K was favoured by pro power amps input impedances in the 1980s.

 

[JohnYang1997]

Sorry on this one, you're mistaken. 2K is ridiculous and you know it, John. You've gone after one metric at the expense of source compatibility and performance. It's a mistake to pass the buck to the source components. A preamplifier is designed to take varying source impedances and levels and buffer them without messing up the performance of the attached components.

47-50K is the defacto input impedance for line sources into a typical preamplifier/integrated line input. It's also the defacto input impedance of an MM RIAA stage and has been this way for many, many decades. Go peruse a few hundred service manuals for amplifiers and preamplifiers and tell me what the "standard" is. I can tell you, it's closer to 50K than 2K.

20-25K was favoured by pro power amps input impedances in the 1980s.

There's no standard. And it works fine with majority of devices. Also it's only the balanced input that has 2kohm. Single ended is 10k which is what majority of modern devices use.
Also there's simply no reason for me to design for every use case possible. It's meaningless. And it's not beneficial.

 

[YSC]

One question is mostly ppl use the pre amp as volume control, interesting to know what the A90 pre SINAD at it's lowest 10-15% setting will be

 

[restorer-john]

There's no standard. And it works fine with majority of devices.

As I said, and you need to understand, 47K-50K has been the defacto standard for many decades. Defacto means it comes into being through use, adherence and general agreement. One guy standing on the sidelines saying 2K is fine isn't going to change a thing.

Literally hundreds of millions of CD players all specified 2.0V out with load impedances required of over 10K minimum to ensure perfectly flat responses and lowest distortion. Tuners require 20-50K. Phono stages need 10K or greater. DVD players need the same.

There is no debate. 2K ohms is ridiculously low and when you've made a few more preamplifiers, you'll understand.

And I like the look of the preamplifier and the input extender. The display looks a bit blocky in the pictures but likely it looks better in the flesh.

 

[Max]

From my point of view, none doubt @JohnYang1997 has made any mistake. He's a skilled electronic engineer and has surely fullfilled the specifications of the product. I know well the possible limitations of a (here relatively) low input impedance but the main question is the market target of the company. Who is the typical buyer of Topping's product ? The "classical" Hifist with bunch of (often less performing) old gears ? I don't think so. Maybe the current typical Hifist isn't anymore the same decades ago, because Topping (and other manufacturers) has opened the market to new customers. Is the 10k RCA input impedance so low it becomes ridiculous ? I don't think so. Is the 2k XLR input relatively low to speak about ? Yes, but so, don't forget it is a tradeoff made for a current changing market. Topping, as company, has certainly made a good choice. Just my opinion