impedence value.? amp to preamp.


if your amp has 15kohm input impedance and your preamp has 20kohm input impedance is this a unworkable combo?
energizer
What you need to look at is the input impedance (the higher the better) of the power amp and the OUTPUT (not input) impedance (the lower the better) of the preamp. The common consent is at least 10 to 1 ratio, i.e. you need the input impedance of an amp to be at least 10 times higher than the output impedance of a preamp.
With a 15kohm input impedance, I would look at an output impedance of 200
ohms or lower, if possible. 15kohm input impedance is on the low side.

Personally, I think the 10 to 1 ratio is too low. When matching for my system, I
now look at a ratio of at least 50 to 1 in order to not have rolled of bass or
treble.

Keep in mind that amps and preamps do not have flat impedance curves. A
preamp with a rated output impedance of 200 ohms can easily have impedance
spikes well above 1000 ohms, and two preamps with nominal 200 ohm output
impedance specs can have wildly varying impedance spikes. Therefore, IMO
the value of the impedance spike should be carefully considered...even more so
than the rated output impedance...when matching gear. John Atkinson's
Measurements section of Stereophile equipment reviews is very helpful when
examining a component's impedance curve.
Both Sidssp and Tvad have fairly well hit the nail. FWIW many tube pre-amps have a 600 ohm output impedence (my ARC SP10 does, and recommends a minimum amp input impedence of 20K ohms). As common are some with output impedences of 3500 ohms. (Three other tubed pre-amps I have have output impedences of 3500 ohms.) None of them really sound all that good with amps with less than 47K input impedence and one even likes 100K best.

Tvad's comments about impedence being a moving target is right on, and you need to start by knowing the output impedence of your Bottlehead (I tried to look it up but couldn't find it).

FWIW.
problem is iam not using my bottlehead.i am considering a passive pre. but i cant afford bent, sonic euphoria,canary ect.
Passive pre-amps/attenuators etc also have output impedence, usually varible, and require very careful IC matching to get a good result.
The output impedance of a resistor based attenuator will not vary with frequency so it's not a concern - assuming there's enough signal left to reach the load to get the volume level you desire.
The concern of resistor based attenuator is that the output impedance changes
a lot depending on the setting. That means when matched with a low input
impedance amp, your system may sound very different as you turns the volume
up and down.
The problem with a lot of passives is that the input impedance of the amp interacts with the cable capacitance and the source impedance of the volume control.

The result is that there is reduced bass and impact at low volumes. Passives really work best when they are built-in to the amplifier.
05-05-09: Sidssp
The concern of resistor based attenuator is that the output impedance changes
a lot depending on the setting. That means when matched with a low input
impedance amp, your system may sound very different as you turns the volume
up and down.
I'm afraid that I do not quite agree with you. For example, here is one implementation of a resistor attenuator:
http://www.tweakaudio.com/Attenuator%20info.html
The only places where the impedance will be high & is likely to affect the sonics is at the -0.5dB, -3dB, -5dB, -7dB settings. Of these settings the -7dB setting is probably the best as the effective impedance the power amp would see in the 500 Ohms region. This is still high for a 10K input impedance power amp but it just *might* work - on the cusp.
(BTW, a digression: my calculations show the 1M resistor provides -0.008dB attenuation, the 4.99K provides -1.5dB attenuation, the 1.69K provides -4dB attenuation, etc. Am I missing something here? Is he adding some other resistor in series w/ the 1K such as the source output impedance (which is not shown)?)
More often than not, the user is going to be in the -31dB range where the effective resistance of this attenuator will be defined by the resistor to ground - which is a very small value. Thus, for even a 10K input impedance power amp this should not be an issue.
(this is all electrically speaking, which is the focus here. No idea what the in-line attenuator does to the overall sonics - my intent is *not* to discuss that here).
Sorry for being dense, but I don't understand what interactions there would be by adding a resistor bewteen the source and amp. Could you give some examples? Thanks.
hi thanks for your input,i will keep interconnects short .5m,also balanced.longer speaker cables.this should help.into 8 ohms i have325rms. so power is there.also helpful.my source will be two cd transports.one arcam,,and one audio alchemy.thi should give me an edge over a normal cd unit.
05-05-09: Bob_reynolds
Sorry for being dense, but I don't understand what interactions there would be by adding a resistor bewteen the source and amp. Could you give some examples? Thanks.
Bob, 1stly, who is the question directed to? The forum in general OR to any particular member?
2ndly, not sure what you are asking - you mean what are the ill-effects of adding a SERIES resistor between pre & power?
You wrote "source" which I interpret as CDP/tape-deck/reel-reel/TT, computer, etc but the author was talking between pre & power. So, you mean between pre & power OR source & power amp, as you wrote?
Thanx.
First, start with the source impedance. Then add to it the value of your passive. That is now the source impedance (much higher) at low volumes. A tube amp has a high input impedance- 100K is common.

The result is that the high output impedance of the volume control, with the high input impedance of the amp, and the character of the interconnect cable itself will result in drive difficulties at low volume, because the source impedance will be primarily the value of the control. At higher volumes this effect is reduced. Thus it is to your advantage to not have a whole lot of voltage output (3 volts max) out of your source, so that you can run the volume control near the top! Midway down, and the control is already degrading the signal.

Most CD player manufacturers don't get this simple point, and so put **way** too much output from their players. I've seen them as high as 15 volts- making a passive control useless (if you want it to sound right).
To put some numbers on Ralphs comments, for example, one of those inexpensive Creeks, the OBH-12, has an output impedence of 0 to 50K, depending of course on the amount of attenuation used. Sonic effect would be sort of a crap shoot with this type of attenuator I think.
The result is that the high output impedance of the volume control, with the high input impedance of the amp, and the character of the interconnect cable itself will result in drive difficulties at low volume, because the source impedance will be primarily the value of the control.

Atmasphere - the resistor attenuator made by EVS seems to have exactly the opposite problem. I.E. a higher volumes the sound might be degraded. Take a look @ the link I provided in my prev post.
The EVS attenuator has a 1K in series & uses shunt resistors to provide the attenuation. So, for any attenuation setting, isn't the effective resistance of the attenuator the parallel combination of the 1K & whatever attenuation setting resistor?
So, for normal volume settings, I see that the shunt resistors are very small in value thereby making the effective resistance this shunt resistor value.
It appears to me that this EVS resistor attenuator is correctly made & will avoid the lack of drive levels at low listening volumes.
You agree?
Thanks Ralph. I think I understand what you're saying. At low volume settings, the output resistance of the volume control will be high, possibly approaching the input impedance of the amp, and thus, much of the signal will drop across the control so driving longer interconnect cables will become an issue.

But, if the output of the source component is reasonable (i.e., max 3V as you suggest) and the ICs are short low capacitance cables, then there is nothing inherently bad about using a resistor based volume control, right? I mean if you can get the volume control and level you desire, then there should be nothing wrong with a simple attenuator regardless of it being 20K ohms or 50K ohms.
Bombaywalla, it will help a lot, in one way, but now you have a different problem- drivability. There are very few sources that can drive 2.2K without distortion and/or bandwidth loss!

There really is not a good way to execute a passive volume control without running into some sort of problem like this. TVCs work much better, but have their own issues as well, although IMO far less severe. IMO/IME experience if you really want to do it right, an **properly designed** line stage is the only way to go.
Hi Bob, to really get a passive to work you get into trouble when you get over 10K for a value, even with short cables. That's why you are better off having the control inside the amplifier, or else inside the source (both inconvenient), and buffered from the cable. Even with 10K, the effect can still be heard.

There was a lot of talk about buffered PVCs a few years back, but what we are talking about then is an active without gain.
Hi Atamasphere,

There are some preamp designs that put volume pots at the output. What do you think of them?

I just bought a preamp that uses 100K volume control at the output. It works well with my power amp which has a 500K input impedance. However, it sounds thin with another power amp that has 60K input impedance.

Please comment. Thanks.
Atmasphere,

(if you want it to sound right).
you get into trouble when you get over 10K for a value
Even with 10K, the effect can still be heard.

"sound right", "trouble", and "the effect"... that's what I'm trying to determine. What happens when the resistance of the volume control is high (ignoring drive issues)?

I understand the issue with a preamp whose output impedance may be 300 ohms at 1KHz, but 3K ohms at 20Hz. It's the increase in impedance as frequency decreases that causes the bass to roll off and not sound right. But for a resistive volume control, I don't see the problem.

Thanks for your patience.
Hi Bob, no worries! The higher the value of the control, the greater its artifact (and that of the cable) is going to be. If you have a higher input impedance, like Vett93 above, the problem you get into is high frequency roll-off.

The lower the input impedance of the amplifier, the harder it will be to get bass at low volumes.

Essentially you can regard a passive system as a sort of tone control that also affects the volume. Because TVCs have greater control of their output impedance they can offer greater performance in this regard if other factors are also taken care of.