The problem of low input impedance of Topping amplifiers

[philomusique]

I am building up my first audio system with two main input sources: iPhone/iPad streaming and a turntable. I decided to use Topping L70 as a low-cost preamp that also doubles as a headphone amp; for the phono stage I chose the Cambridge Alva Duo due to its good measurement, and Topping E70 DAC to source digital inputs.

I know L70 has a low input impedance at 2k ohm, which has been extensively discussed here in the L70 thread. I thought it should be ok, assuming L70 is designed to pair with E70, and the Cambridge Alva Duo, being a modern solid state phono preamp, should also have low output impedance.

Well...this setup is now in serious reconsideration. I emailed Cambridge support to get output impedance spec of Alva Duo. They came back with a quick answer: 100-600 ohms, depending on input signals. By the 1:10 rule, Alva Duo and L70 don't pair well.

In fact, from what I gathered here, other Toppping amplifiers also suffer the same fate. The A70 Pro, which Toppping advertised to be a preamp too, has 20k ohm input impedance only at its XLR inputs. Its RCA inputs still has the meager 2k ohm input impedance, as measured here. The dedicated preamp Pre90 seems to suffer the same issue for its XLR but not its RCA inputs.

I don't know why Topping designed their preamps (either the double-purpose L70/A70 Pro or the dedicated Pre90) in this way. The low input impedance on any of the inputs seriously limit their usability.

 

[KSTR]

I emailed Cambridge support to get output impedance spec of Alva Duo. They came back with a quick answer: 100-600 ohms, depending on input signals

That's highly unlikely and it is a quite unprofessional answer. The output impedance of a solid-state output (opamp-based for sure) is always fixed. So it's either 100R or it is 600R, and the latter would be considered incompetent by today's standards as there is no technical reason whatsoever to make the output impedance that large.

At any rate, a 2k input impedance should not pose any problems at all. If anything, the level drops a bit (quite a bit if 600 Ohms, though), but that's it.
Proper output stages don't have any issues with load impedances of 2k Ohms at normal signal levels. If they do have issues (significantly increased distortion) they're garbage anyway.

 

[mhardy6647]

The rule of thumb for impedance 'matching' is just that.
Did you try the combination you're (now) unsure of, @philomusique ? Empiricism is always hard to beat.

 

[philomusique]

I have the combination running. I cannot be certain if the impedance “mismatching” causes an issue. I do feel, to my untrained, subjective ears, my turntable does sound a little bit funny. But I’m not sure. That’s the reason I contacted Cambridge Audio support.

I ordered a traditional preamp and I’ll do a subjective testing when the new amp arrives.

 

[philomusique]

That's highly unlikely and it is a quite unprofessional answer. The output impedance of a solid-state output (opamp-based for sure) is always fixed. So it's either 100R or it is 600R, and the latter would be considered incompetent by today's standards as there is no technical reason whatsoever to make the output impedance that large.

At any rate, a 2k input impedance should not pose any problems at all. If anything, the level drops a bit (quite a bit if 600 Ohms, though), but that's it.
Proper output stages don't have any issues with load impedances of 2k Ohms at normal signal levels. If they do have issues (significantly increased distortion) they're garbage anyway.

I don’t have equipment to measure the output impedance of Alva Duo. If someone else can do it, it would be great.

I do feel that in additional to the regular SNR and THD and power measurements, the testing should also cover the “deployability” of a device, like impedance and some other factors.

I have a strong feeling these headphone amps like Topping ones are really designed for a narrow use case paired with their own DACs for computer/smartphone digital source to drive headphones.

 

[Nopiano]

I have the combination running. I cannot be certain if the impedance “mismatching” causes an issue. I do feel, to my untrained, subjective ears, my turntable does sound a little bit funny. But I’m not sure. That’s the reason I contacted Cambridge Audio support.

I ordered a traditional preamp and I’ll do a subjective testing when the new amp arrives.

What turntable and cartridge are you using? Has it sounded not ‘funny’ previously? I ask because if you’re used to digital sources it may well sound funny, as noise and sensitivity to the environment isn’t something that digital suffers from in the same way.

 

[Speedskater]

That's highly unlikely and it is a quite unprofessional answer. The output impedance of a solid-state output (opamp-based for sure) is always fixed. So it's either 100R or it is 600R,

While it's always fixed, it can be any impedance the circuit designer chooses.
Typically in modern well designed components, it's between 50 and 250 Ohms.

 

[restorer-john]

I don't know why Topping designed their preamps (either the double-purpose L70/A70 Pro or the dedicated Pre90) in this way. The low input impedance on any of the inputs seriously limit their usability.

It's all about the SINAD chart for Topping- nothing more.

Their input impedance is ridiculously low, way out of the normal range for HiFi.

That's highly unlikely and it is a quite unprofessional answer. The output impedance of a solid-state output (opamp-based for sure) is always fixed. So it's either 100R or it is 600R, and the latter would be considered incompetent by today's standards as there is no technical reason whatsoever to make the output impedance that large.

At any rate, a 2k input impedance should not pose any problems at all. If anything, the level drops a bit (quite a bit if 600 Ohms, though), but that's it.
Proper output stages don't have any issues with load impedances of 2k Ohms at normal signal levels. If they do have issues (significantly increased distortion) they're garbage anyway.

You don't know what he asked and what they replied to. You are making assumptions that may be quite incorrect. Do you know what the output stage topology is? It may have switched attenuators on it depending on the MC or MM twin front end stages. I can see two relays...

They could also be referring to the HP and RCA output R.

And 2k input impedance for a preamplifier is utter garbage. It's not just level that suffers either. Source THD goes through the roof potentially and FR varies.

Don't go accusing Cambridge of being "unprofessional" without all the facts first.

 

[philomusique]

This is the question that I put to Cambridge Audio's support:
"...I would like to know its RCA out's output impedance, so I can know if it matches the preamp well. My preamp has a lower than usual input impedance."

This is their answer, quoted directly (bold text is copied over from their reply):
"This is around line level, 100 to 600 ohms this can vary depending on the signal itself."

 

[KSTR]

This is their answer, quoted directly (bold text is copied over from their reply):
"This is around line level, 100 to 600 ohms this can vary depending on the signal itself."

Looks like that's *not* literally the answer the design engineer would have given.
The poor support person mixed something up here, there are many hints of him/her not really understanding what they wrote:
An output impedance is not a voltage level so it cannot "be around line level". More importantly, while output impedance may actually change between some fixed settings depending on operating mode (@restorer-john -- though I'm quite sure the relays we can see are for output muting), it cannot "vary depending on the signal itself". If that were true, it would modulate its output impedance with signal voltage and this would be a catastrophic distortion generator (and actually quite hard to implement, such a strange behavior).

So most likely the output is either a constant 100R or a constant 600R. The fact they don't state the output impedance neither in the manual nor on the website is not a good sign. This is a very standard specification item which should not be missing.

They could also be referring to the HP and RCA output R.

Well, we now know that the OP ask explicitly for the RCA output impedance.

And 2k input impedance for a preamplifier is utter garbage. It's not just level that suffers either. Source THD goes through the roof potentially and FR varies.

You need a very(!) lousy output stage for significant THD rise at 2Vrms into 2k which is a whopping demand 2 milliamps rms -- unless they used total junk like TL072 (OK for other stuff but not as line out driver). FR variation, where would it come from? The only scenario I could think of is an way undersized output DC-blocking cap... which would also indeed affect LF THD.

I do agree that input impedance is traditionally specified with at least 10k or even 47k, stemming from the old days. Today (and for the last two decades), any bog standard opamp like 5532 or 33078 and the like can deal with 2Vrms into 2k but of course the lighter the load the better.

It's all about the SINAD chart for Topping- nothing more.

This could actually be true, assuming they use some inverting input circuit topology where the input resistance directly affects noise spec (and gain as well). Not a good design choice at any rate.

 

[solderdude]

Well...this setup is now in serious reconsideration. I emailed Cambridge support to get output impedance spec of Alva Duo. They came back with a quick answer: 100-600 ohms, depending on input signals. By the 1:10 rule, Alva Duo and L70 don't pair well.

The '1/10th rule' only applies to the output resistance of headphone or speaker amplifier outputs and is only of importance when the load has a substantially variable impedance and is low impedance.

For inputs of amps this does not apply as they are ohmic.
The worst that can happen is:

• rolled of lows when the output of the connected device has a low value output capacitor
• When a line-level audio transformer is in the path FR and distortion could be modified
• some attenuation can occur
• the output reaches its current limit (which is highly unlikely and needs to be well below 600ohm) most opamps on the output of a source have no problems driving 1kohm loads but distortion (when reaching TOTL numbers) might take a tiny (inaudible) hit.

A high (> 1kohm) output impedance or a varying one (R2R ladder not buffered) or high one (tube source+ low value output cap) or using very long interlinks that also happen to be high capacitance could modify the frequency response in an audible way.

I don't know why Topping designed their preamps (either the double-purpose L70/A70 Pro or the dedicated Pre90) in this way. The low input impedance on any of the inputs seriously limit their usability.

To get the noise numbers down was stated here by the Topping designer.
The reason is when someone buys gear that ensures low noise and low distortion it is connected to high quality sources with low output impedance and capable of driving at least 600ohm. In other words have a buffered output using good opamps.
That excludes 'boutique' and tube output gear.

 

[MaxwellsEq]

This is is their answer, quoted directly (bold text is copied over from their reply):
"This is around line level, 100 to 600 ohms this can vary depending on the signal itself."

As others have noted, that's quite a strange answer to your perfectly sensible question. I'm not convinced that the person answering your email fully understood output impedance.

Given that the duo has very few user adjustable settings, I'd expect either one answer (for both MM and MC) or two separate answers, one each for MM and MC. Usually, it's a fixed value, or one that varies with frequency, rather than varying greatly with level (assuming the device is operating within sensible parameters.

The 1:10 rule for "impedance bridging" is a "safety" rule - in other words you should be confident of no noticeable interaction between two devices. However, you can get away with 1:5 or even 1:1, assuming there are limited reactive components to the impedance and you don't mind trading away some signal level.

 

[audio_tony]

Looking at the picture of the Alva Duo, it looks as though there might be some decoupling capacitors in the output.
If those caps are low in value, then given the low input impedance of the preamp, there will likely be some bass rolloff and this may be the odd sound the OP is referring to?

 

[solderdude]

Looking at the picture of the Alva Duo, it looks as though there might be some decoupling capacitors in the output.
If those caps are low in value, then given the low input impedance of the preamp, there will likely be some bass rolloff and this may be the odd sound the OP is referring to?

View attachment 408824

If I had to guess (calculated based on the 100-600 remark) the electrolytics are paralleled with film caps total around 15uF.
This means that when connected to 2kohm 20Hz may have dropped 2dB
Paralleling smaller caps to larger value ones is absolutely pointless b.t.w. for coupling caps.

 

[philomusique]

Cambridge Alva Duo has an L-R channel balance control, located in the back of the amplifier. I don't know if this can explain the variable output impedance answer I received from their support.

 

[audio_tony]

Depending on where in the circuit the balance control is located, it could indeed explain the variable output impedance.

It would seem an odd circuit choice though.

 

[mhardy6647]

An output impedance is not a voltage level so it cannot "be around line level". More importantly, while output impedance may actually change between some fixed settings depending on operating mode (@restorer-john -- though I'm quite sure the relays we can see are for output muting), it cannot "vary depending on the signal itself". If that were true, it would modulate its output impedance with signal voltage and this would be a catastrophic distortion generator (and actually quite hard to implement, such a strange behavior).

I suspect the hapless support person was referencing the classic line level impedance of the old days of radio feeds via telephone line -- which was (is) indeed 600 ohm.

 

[Speedskater]

To complicate things, for some unknown reason, Stereophile tests pre-amp outputs into a 600 Ohm load. (as one of it's tests)

 

[philomusique]

I bumped into some measurement data by soundstagenetwork.com showing some amplifiers' damping factor varies across the signal frequency spectrum. Here are the plots of 2 solid-state speaker amplifiers:

Benchmark AHB2

Rotel RT-6000

So I think frequency-dependent output impedance is not uncommon, indicating internal capacitive load.

 

[solderdude]

Cambridge Alva Duo has an L-R channel balance control, located in the back of the amplifier. I don't know if this can explain the variable output impedance answer I received from their support.


View attachment 408868

That would not be frequency dependent in that case.
Also it is highly unlikely that the balance control would be in the output circuit. It would make much more sense to put that before any buffering.

About the damping factor of speaker amps.
These differences are very small and are caused by feedback getting smaller for higher frequencies and or the usage of a small inductor at the output.
For the Rotel with an 8 ohm load the upper treble will have some 0.006dB attenuation assuming a tweeter does not rise in impedance otherwise it will even be smaller.
For the AHB2 the treble will drop 0.25dB at 20kHz assuming the impedance does not rise there.
As far as damping factors go they are really only somewhat relevant for woofers and even there a passive low pass filter will 'f-up' the damping factor even more than the amp will. Even more so when the woofer crosses over at a low frequency.

A high output resistance on a headphone amp can (depending on the headphone) have a more dramatic effect which can change the FR up to several dBs and can be very audible.

In the case of the pre-amp it will very likely be output capacitors (to get rid of DC components) as audio tony suggested. Also there is no varying load impedance.