Question about power amp input impedance

[gtirard]

I've read that SUMO power amp requires a preamp with a high output, and some user complain about a lack of power with some preamps.
SUMO's input impedance: 10 K Ohm
Jad Freer CODA's input impedance: 200K Ohm
Does-it mean that we can use Jad Freer CODA with more preamps that the SUMO, including the ones that have a lower output?

Is there a rule or a math formula we can use to check if a preamp and a power amp will match?
I would have thought the input impedance of the power amp should match the output impedance of the preamp, but we never see this spec published for preamps, maybe it doesn't make any sense? How do we know?

 

[G Aichele]

FWIW, I've used both the Baby Sumo (600w) and Sumo (1000w) amps with various preamps. Never had a problem.

 

[Redbrangus]

Power amps (including the power amp section of bass amps) have high enough input impedances that they don't present a load to the output of the preamp or other device that drives them. The critical factor is whether the preamp is capable of providing sufficient output level (voltage) to drive the amp to its full output voltage. Whether or not a preamp 'works' with a particular amp is dependent on its output level and the input sensitivity of the amp. Typical input sensitivity for power amplifiers generally ranges from 0.775V (0 dBu) to 1.4V (+4 dBu). Preamps intended to be used with the input of a bass amp's internal preamp typically don't provide that sort of output level.

 

[Wasnex]

Power amps (including the power amp section of bass amps) have high enough input impedances that they don't present a load to the output of the preamp or other device that drives them.

I believe the term for this is "impedance bridging." Usually the input impedance of the driven device (load) is multiple orders higher than the output impedance of the driving device (source). AFAIK when the load impedance is >10 times the source impedance, the circuit is said be bridged. Essentially the source behaves as if there is no load present (open circuit).

A consequence of bridging is multiple devices can be driven off a single source with little to no signal degradation.

Is there a rule or a math formula we can use to check if a preamp and a power amp will match?

Preamps and power amps above a certain quality level tend to have specs for this. Compare the max output level for the preamp to the sensitivity rating of power amp. The sensitivity rating of an amplifier is the level required to drive it to full power. The preamp's max output level must be equal to or greater than the sensitivity rating. I assume it's preferred for the preamp's max output to exceed the sensitivity rating by some degree. I would assume something like 3 to 6dB would be enough, but this is a guess.

If the input impedance of the amp is too low, it can load down the output of the preamp, and thus reduce available voltage. So if the preamps output level is exactly the same as the amp's sensitivity rating, it could be problematic if the amp has an unusually low input impedance.

 

[Wasnex]

These are the output specs for my old A&H MixWizard 16

I believe the 4dBu for the LR main Out is the nominal level. AFAIK the <75 ohms is the output impedance.

AFAIK this means 27dBu is available as long as the load is 600 ohms or higher.

QSC PowerLight 3 has 10K input impedance and they have three input gain settings: 26db, 36dB, and 1.2V

Take a look at the column for the PL325

• For 26dB gain setting, sensitivity is 3.28V
• For 32dB gain setting, sensitivity is 1.6V
• For the 1.2V setting, 1.2V is the sensitivity rating. Gain is 34.5dB
  • Gain means the internal gain of the power amp--In other words 34.5dB of gain or amplification is required to raise 1.2V to ~63.3V which is approximately 500W at 8 ohms.
    • Here's the math
      • Output Voltage for 500W at 8 ohms is ~63.246V
      • Input Voltage = 1.2V
        • Gain = 20Log(~63.246V/1.2V) = ~34.5dB

 

[agedhorse]

I've read that SUMO power amp requires a preamp with a high output, and some user complain about a lack of power with some preamps.
SUMO's input impedance: 10 K Ohm
Jad Freer CODA's input impedance: 200K Ohm
Does-it mean that we can use Jad Freer CODA with more preamps that the SUMO, including the ones that have a lower output?

Is there a rule or a math formula we can use to check if a preamp and a power amp will match?
I would have thought the input impedance of the power amp should match the output impedance of the preamp, but we never see this spec published for preamps, maybe it doesn't make any sense? How do we know?

You are quoting g the input impedance of the CODA, yet referring to the output impedance when it comes to interfacing to a power amp.

General rule of thumb is that the output impedance of the preamp output should be less than 1/10th the input impedance of the power amp.

 

[logdrum]

Jad Freer CODA's input impedance: 200K Ohm

That is too low. Are you sure you are not talking output impedance. which is very high 1 megaohm is typical and 5 megaohm is good for piezos and some piezos like 10 megaohm. 500 kOhm is the lowest limit that a variable input impedance preamp like summit audio provides and is intended for magnetic pickups. Typical output impedance like my ZIO is 75 to 100 ohms

 

[bherman]

A few years ago I purchased a Sumo 600 with the hopes of using it with a Mesa Subway+ preamp. The output level from the preamp was not adequate to drive the power amp. I had exchanged pms with both @agedhorse and @Wasnex both of whom were very helpful. I do not recall the specific metrics involved, but ultimately determined that a typical bass pedalboard preamp (such as the Subway +) was not a good match for the amp, so I sold the amp and moved on.

 

[agedhorse]

A few years ago I purchased a Sumo 600 with the hopes of using it with a Mesa Subway+ preamp. The output level from the preamp was not adequate to drive the power amp. I had exchanged pms with both @agedhorse and @Wasnex both of whom were very helpful. I do not recall the specific metrics involved, but ultimately determined that a typical bass pedalboard preamp (such as the Subway +) was not a good match for the amp, so I sold the amp and moved on.

This doesn't sound right since the Subway + pedal can easily drive line level (way more than +4dBu) into a 10k balanced input all day long.

There may have been something else going on with the amp though.

 

[agedhorse]

That is too low. Are you sure you are not talking output impedance. which is very high 1 megaohm is typical and 5 megaohm is good for piezos and some piezos like 10 megaohm. 500 kOhm is the lowest limit that a variable input impedance preamp like summit audio provides and is intended for magnetic pickups. Typical output impedance like my ZIO is 75 to 100 ohms

Anywhere from 500k to 1M is pretty typical but 220k is not that uncommon either.

Piezos typically perform best right around 1M but there are exceptions for very small crystals that work better with up to 5M. Beyond 5M, IME there's no benefit except for marketing a higher number and there are some significant negatives to this as well (including higher noise floor and higher cable tribo-electric noises).

There are some benefits in using 1M, including a little higher damping which can tighten up a sloppy piezo and reduce the tendency to feed back or become barky-quacky. It's all about balancing the various tradeoffs.

 

[logdrum]

Anywhere from 500k to 1M is pretty typical but 220k is not that uncommon either.

Thanks for the clarification. I thought that you do not want to go below 500k, Yeah the 10Mohm was just a setting on older AER and Headway preamps and I think the new Headway he has a 20 M setting - he does not lean onto that feature though in his advertising which is also almost absent.

The AERs are now just 2.2 -- The old BP 100 pickups as well as underwood sounded better with the 5 Mohm though but everything else like Fishman and David Cage are fine with 1M, But we are talking double bass at this point and many folks have an acoustic preamp of some sort, I always had in that way, I can use any amp

 

[SactoBass]

I've read that SUMO power amp requires a preamp with a high output, and some user complain about a lack of power with some preamps.
SUMO's input impedance: 10 K Ohm
Jad Freer CODA's input impedance: 200K Ohm
Does-it mean that we can use Jad Freer CODA with more preamps that the SUMO, including the ones that have a lower output?

Is there a rule or a math formula we can use to check if a preamp and a power amp will match?
I would have thought the input impedance of the power amp should match the output impedance of the preamp, but we never see this spec published for preamps, maybe it doesn't make any sense? How do we know?

As I understand it, there are two things to consider when connecting a preamp pedal to a power amp such as the Sumo or Coda (or any other power amp): signal loading, and voltage requirements. Let's talk signal loading first.

I have a ShiftLine Olympic (SLO) MkIIIS preamp pedal. The 1/4" out jack has a 1k (1000) ohm output impedance. The rule of thumb is for the receiving device to have at least 10 times the input impedance of the sending device's output impedance to avoid signal loading. So the Sumo power amp's 10,000 ohm input impedance would work with my SLO preamp with respect to signal loading, but just barely. The 200,000 ohm input impedance of the Jad Freer CODA power amp is waaaay higher and would therefore work great with my SLO preamp with respect to signal loading.

Okay, that's signal loading.

Now let's talk about voltage. A preamp needs to have enough output voltage to drive the power amp to full power.

My SLO preamp's 1/4" out jack is rated at +20dBu maximum, which is around 7.75 volts. Since most power amps have an input sensitivity of somewhere between .775 volts and 1.4 volts to reach max power, my SLO will easily drive most power amps to full power with lots of room to spare. Of course, not all preamps are created equal. So it is important to know the output voltage of whatever preamp is being used, and the input sensitivity of the power amp.

Hope that info helps!

 

[gtirard]

As I understand it, there are two things to consider when connecting a preamp pedal to a power amp such as the Sumo or Coda (or any other power amp): signal loading, and voltage requirements. Let's talk signal loading first.

I have a ShiftLine Olympic (SLO) MkIIIS preamp pedal. The 1/4" out jack has a 1k (1000) ohm output impedance. The rule of thumb is for the receiving device to have at least 10 times the input impedance of the sending device's output impedance to avoid signal loading. So the Sumo power amp's 10,000 ohm input impedance would work with my SLO preamp with respect to signal loading, but just barely. The 200,000 ohm input impedance of the Jad Freer CODA power amp is waaaay higher and would therefore work great with my SLO preamp with respect to signal loading.

Okay, that's signal loading.

Now let's talk about voltage. A preamp needs to have enough output voltage to drive the power amp to full power.

My SLO preamp's 1/4" out jack is rated at +20dBu maximum, which is around 7.75 volts. Since most power amps have an input sensitivity of somewhere between .775 volts and 1.4 volts to reach max power, my SLO will easily drive most power amps to full power with lots of room to spare. Of course, not all preamps are created equal. So it is important to know the output voltage of whatever preamp is being used, and the input sensitivity of the power amp.

Hope that info helps!

Thank you so much for this very clear explanation, that's exactly what I wanted to understand.

 

[agedhorse]

Power amp’s 10k input impedance is plenty fine with a preamp’s 1k output impedance. No problem there.