How do you match a pre amp to a power amp?

QuestionsCategory: QuestionsHow do you match a pre amp to a power amp?
El Hefe asked 1 year ago

Hello,
What are the rules or guideline to match a pre amp and power amp if both of them are of different brands or series? Some say we need to ensure the gain is matched. Some even mix between a tube pre amp and solid state power amp. Care to share your experience?

2 Answers
lam seng fatt answered 1 year ago

More often than not, you get better results when you use a preamp and a power amp from the same manufacturer.
If you use a preamp and a power amp from different companies, it can be a challenge to find a combination that matches well.
Having said that, many audiophiles have told me that it is easier to match a tube preamp with a solid-state power amp than it is to match a solid-state preamp with a valve power amp. Usually, a solid-state preamp/tube power amp combination will not sound good.
A tube preamp will sound good with a tube power amp or a solid-state power amp. Not only that, but a tube preamp/solid-state power amp will give you a hybrid sound of the best of both worlds – the warmth of tubes and the punch and dynamics of solid-state.
If you want a preamp that is versatile, go for a good tube preamp. The disadvantage is that you will have to replace the tubes after a few years. Bear in mind that changing the tubes will also change the sound. But tube-rolling is what the tube diehards love to  do anyway.

El Hefe replied 1 year ago

Thanks Lam. I can agree to the statement that tube pre amp + SS power amp sounds more enriching compared to SS preamp + tube power amp.

The hybrid concept is kind of unique and I did have a go with one from a local DIYer.

Nevertheless, does this mean that ebgineering wise, there is no incompatibility issue to mix and match pre and power amp from different makes but it may not sound that good.

lam seng fatt answered 1 year ago

In terms of electrical parameters, I think gain matching is not an issue. The main problem is the output impedance of the preamp and the input impedance of the power amp. These impedances must match well.
 
Another issue is the output voltage of the preamp and the input sensitivity of the power amp. If the output voltage is too high, it may overload the input of the power amp.
 
Some DAC/preamps (like the dCS) have adjustable output voltages. Strangely enough, people who have played the dCS Puccini CD/SACD player-cum-DAC-cum-preamp (actually it is a digital volume control), which has 2V or 6V output, say the higher setting sounds better even though you would initially think the 6V would overload the power amp. So if you have a preamp or DAC/preamp with variable output voltage, you have to experiment.
 
For the benefit of other audiophiles, I managed to find this informative article from http://www.decware.com/paper55.htm. Please read this:
 
To select an ideal preamp you must know two things:

  1. The output voltage of your source (most stock CD players are 2 volts)
  2. The input sensitivity of your amplifier (most amplifiers are around 1 volt)

If your CD player or other source has an output level less then or greater then 2 volts, you need to know this. On the same note, if you power amplifier has an input sensitivity less then or greater then 1 volt, you need to know that too. Once you know for sure what your voltages are you are well on your way to picking the right preamp.
Lets look at these two terms so we understand exactly what they are.
The output voltage of your source is a constant level, and does not change unless your source has a “variable output”. This 2 volts of signal (music) drives the input stage of your power amplifier or it drives the input stage of your preamp which in turn drives the input stage of your power amp.
The input sensitivity of an amplifier simply means how many volts are required to bring the amplifier to full power. Any amount of voltage beyond this figure will make your amplifier try to put out more power then it actually has, the result is called “clipping”.
So it stands to reason that in all cases a preamp is used to control the voltage from the source. You can figure that when the preamp volume is all the way down, you have ZERO volts – hence no sound from the amplifier. As you turn the volume knob up the voltage increases as does the sound you hear from your amp. The ideal working range on a volume control should be somewhere between 1/4 and 3/4 of the way up to hit your normal listening level . This means your preamp would actually never be adding any voltage to it’s input signal – also known as “gain”.
When do you want “gain” in a preamp? There are two reasons you might want gain in a preamp:

  1. When your power amplifier needs more then 1 volt to come to full power.
  2. Or when your source has less then 2 volts of output.

Some amplifiers can need up to 5 volts to bring them to full power, Decware amps are all set to reach full power with 2 volts. On the flip side I have seen amplifiers that needed only ½ volt to come to full power.
Some modified CD players could have less then 2 volts, and some DACs could also have less then 2 volts. (Although most DACs have at least 2 or slightly more volts – in some cases up to 5)
Vinyl lovers know that it can be sometimes difficult to find a cartridge and phono stage combination that they like with a full 2 volts of output. I’ve seen many with closer to 1 volt.
In the case where your source has say only 1 volt, and your amp has an input sensitivity of say 2 volts you will absolutely need a preamp with some gain, otherwise you’ll never be able to play your amplifier near as loud as it could go. Even if you have super high efficiency speakers and listen at very soft levels, your music will still suffer from a lack of dynamics and weight.
Once you make the determination that you either need gain or don’t need gain, you will have narrowed down your choices considerably. In either case, gain or no gain, there is one more thing to consider and that is called “impedance”.
All sources and preamps have what is called “output impedance”. Think of it as the units ability to drive difficult loads, like might be experienced with super long interconnects, or obnoxious amplifiers.
On the flip side, all amplifiers have what is called “input impedance”. Think of it as the difficulty level the amplifier imposes on the preamp or CD player.
As a general rule, the lower the output impedance is, the better it will drive difficult loads. And, the higher the input impedance is the less difficult it is to drive. A good example of a happy situation is a preamp with an output impedance below 1000 ohms driving an amplifier with an input impedance of 100,000 ohms.
Most stock CD players have a fairly low output impedance, but unfortunately power amplifiers are all over the place, ranging between 10,000 ohms and 500,000 ohms. The most common figure is around 50Kohms.
50Kohms is a load that most sources and preamps can drive without problems. The exception might be when a preamp with a fairly high output impedance trys to drive a 50Kohm amplifier through extra long interconnects. The result can often be either rolled off bass response or lack of dynamics or both. You don’t want it to sound thin, so you run short interconnects. If the preamp had an advertised low output impedance then it wouldn’t matter if the power amp was clear on the other side of the house, it would still have solid dynamics and weight without rolled off bass response.
 

El Hefe replied 1 year ago

Thanks Lam for the in depth explanation. While there are a number of enthusiasts that like to mix the pre and power amps but I am quite sure, most people would go for the same brand/models of pre and power to avoid mismatch between them.