Amp preamp impedance matching...can anyone explain?
Sorry for reviving this important thread. In the posts above, we see conversation about output impedance of (tube) preamps that have a coupling capacitor (the fact that, most likely, the output impedance shall be highest at 20Hz). What about impedance matching for tube preamps like the VAC Renaissance Mk V that have transformer coupled output (and input) stages? |
Transformer coupled output is an excellent approach, especially for tube preamps which otherwise contend with the noted issues of output caps and rising impedance. The Ren V claims an even 150 ohms or less over the entire music bandwidth, and so should have no problem driving any SS or tube amp. Using transformer coupling for both inputs and outputs also frees up the preamp circuit to use “only” 2 tubes, and handles SE to balanced (and the reverse) conversions “for free”. |
That looks to be true, but if you look at the pre-amp specs, the Ren V wants to see a recommended output load of 300 ohms. Most solid state pre-amp's have input ranges of 10K to 47K (using those figures as general examples), which will not provide this load. As the radio broadcast industry segued from transformer outputs, to op amp inputs/outputs, many transformer devices (audio gear) incorporated selectable 600 ohm terminating resistors, so that you could load the transformer output (a perfect example was 600 ohm transformer outputs) if feeding a high impedance input. I think many tube pre-amp owners need to be aware of transformer output loading when feeding a solid state high impedance input. |
@dpop it specifies a recommended load impedance of greater than 300 ohms. That includes loads like 10K, 50K, 500K, etc. This is not like matching taps on power amp transformers, where maximum power transfer is the goal. Here we are only concerned with transferring voltage signal, where Ohm’s law dictates a good transfer. That means you want as low an impedance on the source side (150 ohms being very low for a preamp, especially at 20 Hz), and a “sufficiently high” impedance on the amp side (300 ohms being far lower than typical). It is reasonable to aim for a load impedance 20x (or more) greater than the source impedance to minimize losses caused by voltage division. |
I totally understand the concept of solid state low impedance op amp output feeding a solid state op amp high impedance input. No questions there, but these transformer outputs (if that’s what the Ren MK V has) have confused me in the past. There’s no doubt in my mind about terminating 600 ohm transformer outputs before feeding them to solid state op amp inputs. That’s definitely necessary, but I’m not 100% sure in this situation if a 300 ohm termination is needed. I suppose Renaissance technical support could answer that question. |
