# tube and transistor amp impedance

#### veenife

Joined Jan 16, 2014
42
why do tube amplifiers have a bigger impedance than trasistor amps?

what is the real cause of that?

#### alfacliff

Joined Dec 13, 2013
2,449
tubes have grids to control current, they are voltage operated divices, transistors use current, tbe grids do not usually draw any current.
input impedance for tubes is much higher than transistors.

#### veenife

Joined Jan 16, 2014
42
what are grids?

#### LvW

Joined Jun 13, 2013
840
veenife,
I suppose, you speak about the INPUT impedance, correct?
Furthermore, you should discriminate between FETs and bipolar transistors (BJTs).

I think, one can say that tubes and FETs - more or less - show similar behaviour (large input impedance - mostly capactive).
However BJTs have a pn junction at its input (like an open diode) that allows a certain input current. Therefore, the input impedance (mostly resistive) is relatively low (some kohms) - in comparison with tubes/FET`s.

#### veenife

Joined Jan 16, 2014
42
okay... it seems to be a very complicated subject....

i was refering more to the amps output impedance actually.... i notice that transistor guitar amps usually can be plugged to any speaker... sometimes they not even tell you the value of the output impedance.... its just one out and no big thoughts need to be made upon the conncetion...

but with tube guitar amps on the back you can get sometimes even 3 output choices with different impedance values like 4, 6 or 8.. sometimes 16 ohms....

so all that i suppose is because with transistor amps the output impedance is anyway so small that one doenst need to worry about it....

but yeah... any help is welcome.. i still didnt quite understand why this is like that....

#### LvW

Joined Jun 13, 2013
840
At least you have learned now that it is important to be very exact in your request. Just mentioning "impedance" while speaking about 4-pole circuits is not sufficient.

#### alfacliff

Joined Dec 13, 2013
2,449
the output impedance choices are set by the output transformer in the amp. the type of amp tube or solid state, dosnt mater, only what the output transformer was designed to do.

#### veenife

Joined Jan 16, 2014
42
okay.. cool, so for each output there is a different transformer that increase or decrease the impedance for the apropriate matching... ???

nice....

but coming back then to input impedance of tube amps...

ive read that tube input impedance are more frequency dependent than transistors, because trasistor have a more of resistive impedance...

then is it tube impedance more of a capacitive or iinductive impedance???

i guess this also applies for output impedance... maybe thats why the possibility of different impedance choices by tube amp???

can any one add more words to that... i mean clarify this a bit better ???

#### alfacliff

Joined Dec 13, 2013
2,449
tubes have capacitive inputs, but only in the picofarad range. the out put impedance is determined by the otput transformer in tube amps, and may have several taps to present differit impedance to loads. the tubes themselvs have a fairly high impedance determined by the supply voltage and current the tube draws. for instance it might ahave a plate impedance of 5k ohms.

#### LvW

Joined Jun 13, 2013
840
veenife,

sorry, but again I must note that your question is not yet clear.
1.) Are you interested in input or output impedances or both?
2.) Is your request related to the active unit (tube, transistor) alone or within an amplifier circuit? This makes a big difference!

#### MikeML

Joined Oct 2, 2009
5,444
Actually, solid state amps have a lot of internal negative feedback, so approximate a voltage source, so don't care if the speaker impedance is 2, 4, 8 or 16 Ohms, or if no speaker is connected at all.

Tube amps dont have much (if any) negative feedback, and do care about matching the speaker impedance to the winding on the output transformer. Most tube amps have multiple taps on the output winding, so select the one that most closely matches the speaker impedance.

#### crutschow

Joined Mar 14, 2008
23,149
Tubes are low current ,high voltage devices so require an output transformer to convert that to the high current, low voltage that speakers require. These transformers typically have 4, 8, and 16 ohm output taps to get the maximum power out for the particular speaker impedance you are using.

Transistor are high current, low voltage devices (relatively) so can be directly coupled to the speaker, avoiding the transformer. Transistor amplifiers do have an optimum impedance load they can drive for maximum power but can still efficiently drive other value loads, although at a somewhat lower max power. This is related to output voltage and current limitations, not from the output impedance value. I have seen a transistor audio amp from McIntosh with an output transformer for optimum power output for any speaker impedance but that is rare.

As Mike noted transistor amps also generally have more negative feedback then tube amps can tolerate to give a much lower dynamic output impedance, typically much less than an ohm.

#### alfacliff

Joined Dec 13, 2013
2,449
MikeMl, I have had to fix a lot of car stereos that were turned on without speakers, or only one speaker with blown oututs. lately, the designs have improved so that they dont do that as often.

#### crutschow

Joined Mar 14, 2008
23,149
MikeMl, I have had to fix a lot of car stereos that were turned on without speakers, or only one speaker with blown oututs. lately, the designs have improved so that they dont do that as often.
That's a puzzle. Solid state amps are normally immune to open circuit output. They may fry though if the output is shorted.

#### THE_RB

Joined Feb 11, 2008
5,430
It's very common to blow a car amp chip if the speakers have an intermittant connection, like a loose speaker wire and vibration from a moving car etc.

But they should never blow from running with no speaker, it was one of my standard tests for car amps when I was a repairer.

#### veenife

Joined Jan 16, 2014
42
LvW veenife,

sorry, but again I must note that your question is not yet clear.
1.) Are you interested in input or output impedances or both?
2.) Is your request related to the active unit (tube, transistor) alone or within an amplifier circuit? This makes a big difference!
hey, thanks for the replys, its all getting much clear now....

but

LvW,

yes im interested in both, input and output impedance... mostly any thing that is related to mic preamps or guitar amplifiers!

but a little of tube and transistor as units is also welcome...

#### veenife

Joined Jan 16, 2014
42
okay, ive been reading further more on the topic... i got some new brainmess going on again...

here in the forum "LvW" mentioned that tube input impedance is mostly capacitve... "alfacliff" said that the capacitive impedance goes on the picofarad range.... (which is very small).... in other places im reading that the input impedance from tube amp is close to the grid's resistor, which is something like 1 Megaohm... (i suppose that mean that this is a resistive impedance) ...

so what is actually right?

#### vk6zgo

Joined Jul 21, 2012
677
Most Solid State Amplifiers do not use output transformers,whereas tube Amplifiers do.

Many Solid State Amps are quite forgiving as far as speaker impedance is concerned.

It is far easier to design for transformerless output with Solid State,than with tubes,so the classic tube Amplifier design uses an Output Transformer.

Some of these transformers are tapped for varying speaker impedances,but many are not.

Negative feedback was certainly used extensively with 1950s tube HI-FI Amps.
Ideally,the negative feedback loop incorporates the speaker side of the transformer,but this can cause instability if the feedback changes to positive at ultrasonic frequencies.

These transformers required special design & winding techniques to avoid phase reversal.