Amp / Pre-amp Impedance Matching

[dminches]

Hopefully this question makes sense. I have an Ayre VX-R 20 amp and want to get a preamp to replace my Ayre K-5xe MP. The main reason why I am replacing the pre-amp is that I want one with a balance control since my seating position is off center and I don't have much flexibility to move the speakers.

Should I be paying close attention to the output impedance of the pre-amp? The K-5xe output impedance is 55 ohms per phase.

I have been looking at the BAT Vk-43SE which has an output impedance of 200 ohms per phase. I am also looking at the Bryston BP-26. I am not sure what the output impedance is for that unit.

The input impedance of the Ayre VX-R 20 is 1 Mohm per phase.

 

[K3RMIT]

aside of end sound yes absolutely may I ask what is the amps input imp and cable run type se or bal
the point is low preamp output imp to high input imp of amp. A general rule is 10 to 1 ratio with pre being 1 amp 10. this has a almost no effect of loading the preamp. some may say even more. i have a krell 700cx amp it’s input is 100 ohms
very low for an amp
it needs a preamp max 10 ohms output. i had to buy a krell preamp to match.
same goes for headphones to headphone amp. if the amp used is 10 ohms the headphone must be much higher or the tone is off badly.
i was going to try a imp iso transformer to make up the imp issue but I was advised it still would be bad.
I hope others here comment back I for one like real world experience beyond just theory.
ps the 2 preamps I had did not play well

 

[dminches]

aside of end sound yes absolutely may I ask what is the amps input imp and cable run type se or bal
the point is low preamp output imp to high input imp of amp. A general rule is 10 to 1 ratio with pre being 1 amp 10. this has a almost no effect of loading the preamp. some may say even more. i have a krell 700cx amp it’s input is 100 ohms
very low for an amp
it needs a preamp max 10 ohms output. i had to buy a krell preamp to match.
same goes for headphones to headphone amp. if the amp used is 10 ohms the headphone must be much higher or the tone is off badly.
i was going to try a imp iso transformer to make up the imp issue but I was advised it still would be bad.
I hope others here comment back I for one like real world experience beyond just theory.
ps the 2 preamps I had did not play well

The input impedance is 1 Mohm.

 

[cpcat]

One megaohm is extremely high and should match well with just about anything.

 

[Alrainbow]

A low output imp also helps in driving longer cable runs as well.

 

[AMR / iFi audio]

Hopefully this question makes sense. I have an Ayre VX-R 20 amp and want to get a preamp to replace my Ayre K-5xe MP. The main reason why I am replacing the pre-amp is that I want one with a balance control since my seating position is off center and I don't have much flexibility to move the speakers.

Should I be paying close attention to the output impedance of the pre-amp? The K-5xe output impedance is 55 ohms per phase.

I have been looking at the BAT Vk-43SE which has an output impedance of 200 ohms per phase. I am also looking at the Bryston BP-26. I am not sure what the output impedance is for that unit.

The input impedance of the Ayre VX-R 20 is 1 Mohm per phase.

The general rule of thumb is that the output impedance of a previous stage should be at least 10 times smaller than the next stage's input impedance. So with the popular 20k input impedance of many amps, we need a maximum 2k output impedance source.
Life isn't that easy, sadly. We need to factor in the cables and their capacitance. My rule of thumb for that is that this difference should be at least in the hundreds.

In your case, this requirement is easily fulfilled.

 

[divertiti]

The general rule of thumb is that the output impedance of a previous stage should be at least 10 times smaller than the next stage's input impedance. So with the popular 20k input impedance of many amps, we need a maximum 2k output impedance source.
Life isn't that easy, sadly. We need to factor in the cables and their capacitance. My rule of thumb for that is that this difference should be at least in the hundreds.

In your case, this requirement is easily fulfilled.

I've heard that 10x rule too, but can you elaborate on why? Theoretically, isn't maximum signal transfer happening when both impedance are the same?

 

[Atmasphere]

I've heard that 10x rule too, but can you elaborate on why? Theoretically, isn't maximum signal transfer happening when both impedance are the same?

No. You are confusing 'characteristic impedance' with simple impedance. Characteristic impedance is something you deal with when working with transmission lines, such as a connection between a transmitter and an antenna.

When a source is driving a load, it puts out a voltage. You want the voltage to be absorbed by the load. When the load impedance is reduced, more of the output voltage is absorbed by the source (preamp) as well. 10x is high enough that the voltage loss is negligible.

 

[Alrainbow]

I've heard that 10x rule too, but can you elaborate on why? Theoretically, isn't maximum signal transfer happening when both impedance are the same?

the ten times is a basic ohms law eqation . if you put a 10 ohms resi in parralell witha 100 ohm the effective res is still 10 ohms , not exactly but very close .

 

[DonH50]

What Ralph said. Maximum power transfer occurs when source and load are matched, but that is not what we want in this case. Since the receiver (load) is working with the voltage input level, not power, we want to minimize the voltage loss (reduction) from the source, That means low source impedance and high load impedance.

A >10:1 ratio (20 dB) means less than 10% voltage loss from the source and is a rule of thumb.

 

[Alrainbow]

No. You are confusing 'characteristic impedance' with simple impedance. Characteristic impedance is something you deal with when working with transmission lines, such as a connection between a transmitter and an antenna.

When a source is driving a load, it puts out a voltage. You want the voltage to be absorbed by the load. When the load impedance is reduced, more of the output voltage is absorbed by the source (preamp) as well. 10x is high enough that the voltage loss is negligible.

true but not always lol .

 

[Atmasphere]

true but not always lol .

Hm. I'm not aware of exceptions.

About the only thing that can get you into trouble is the output impedance at 20Hz. That is often considerably higher than the 1KHz reading, which is the one that show up on spec sheets. If the 20KHz output impedance goes high enough it can result in a loss of bass impact. We got around that problem by use of a direct coupled output circuit we patented.

 

[Alrainbow]

What I meant is the interaction of interconnects as sound is the goal. i was not aware of bal versus se on low freq subs. Up until I was told not to use bal on my woofer towers witch are self powered.
my set up is the Infinty IRS V
when I had my cross overs done both for mid tweets and woofer boxes
I was told to leave the woofer towers as se
I did ask and think it was low freq imp of some kind. again thanks for input always

 

[Alrainbow]

What Ralph said. Maximum power transfer occurs when source and load are matched, but that is not what we want in this case. Since the receiver (load) is working with the voltage input level, not power, we want to minimize the voltage loss (reduction) from the source, That means low source impedance and high load impedance.

A >10:1 ratio (20 dB) means less than 10% voltage loss from the source and is a rule of thumb.

Krell had there own design on this making the amp as a source lower them there preamp
if you used there own output
this they claimed reduced greatly the effects of interconnects

 

[DonH50]

Krell had there own design on this making the amp as a source lower them there preamp
if you used there own output
this they claimed reduced greatly the effects of interconnects

IIRC, the Krell CAST system uses current-mode driver (high-impedance) and receiver (low-impedance) circuits. One idea (claim) being that current-mode interconnects pick up less noise than voltage-mode links. Essentially the output of the preamp is a current source, ideally infinite impedance, and the amplifier input a current sink which is optimally 0-ohm impedance. Current mode is often used in IC bias circuits because coupling on-chip does tend toward voltage noise moreso than current noise. A number of my (RF/mW) designs operated in current mode as it provided greater bandwidth and decently low distortion, albeit with higher noise. The current source is also sensitive to loading; you are depending upon the load's very load impedance to counter that. There are some very high-speed (up to Gb/s) signal interfaces that are current-based.

The trade is less clear (to me) for an audio link given high-current power and speaker lines that may induce noise via current coupling, and there is the argument that you've just moved the high-impedance (more sensitive) point from the load to the source. There is also the issue that you still have to convert to voltage someplace, and, while current-mode amps tend to be more intrinsically linear than pure voltage-mode amps, current amplifier circuits tend to have much less gain making it difficult to apply feedback to improve their performance. (Yes, I realize the trend toward "no feedback", I just don't agree with it. But, IIRC, Krell claimed "no feedback" in their circuits, accomplishing low distortion with lots of bias current, then -- I think -- varied dynamically to keep from burning down the house.) The scheme seemed to work, but it has been many years since I listened to a Krell component. I have no reason to doubt their implementation.

Special cases aside, the vast majority of preamps today output a voltage, and the amplifier inputs expect a voltage, so I think the argument is valid even though you can always find an exception.

IME/IMO - Don

 

[Alrainbow]

Very kind of you to reply
I’m using a krell FBP700 cx amp
whwn I bought it
it came a a package with a ktc preamp. at the time I was using a ML no26 with my ML no33 amps
now the ML preamp did not have a ultra low output imp
but did have a bal output to use well. My interconnects are a bit long at 30 feet in total
the krell sounded horrible
I then tried the krell ktc and it was wonderful.
I even tired there cast as well. All was good.
i wondered why
I then read the specs were 100 ohms Input to amp. not 10k as some posted.
So the ktc is needed
I wonder how many bought a used 700cx or even knew and hated it
I rebuilt both amp and preamp
no model below or above had such a low input imp
I wonder why he made this one soo low

 

[AMR / iFi audio]

Krell had there own design on this making the amp as a source lower them there preamp
if you used there own output
this they claimed reduced greatly the effects of interconnects

The lower input impedance of a receiver minimises the effect of cables. Cable capacitance creates a filter with input impedance and the higher the impedance is, the lower in frequency this filter works.

 

[Alrainbow]

The lower input impedance of a receiver minimises the effect of cables. Cable capacitance creates a filter with input impedance and the higher the impedance is, the lower in frequency this filter works.

it is exactly what krell claims . there was very little change from usinf xlr cables and a compare to krell cast to me . but the amp input imp is a tough amp to drive . needs current and even when i tried a inline iso transfomer to better match the imp mismatch with my ml no26 it still was bad .

 

[DonH50]

If you have a hi-Z source and low-Z load or lo-Z source and hi-Z load the net effect due to cable capacitance on system bandwidth is roughly the same, and for audio is negligible unless you have incredibly long lines. See e.g. https://www.whatsbestforum.com/threads/interconnect-frequency-response.3185/#post-47789

 

[Atmasphere]

this they claimed reduced greatly the effects of interconnects

Another way to reduce the effect of the interconnects is to use balanced connections. But to do this the equipment has to support the balanced line standard (largely ignored in high end audio) also known as AES48. This is the approach we took with our preamps back in the 1980s. It works.

The balanced line system is very good at eliminating interconnect cable artifacts ('effects' per your post), and literally ushered in the era of hifi back in the early 1950s prior to stereo.