Amp preamp impedance matching...can anyone explain?

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”. 

@mulveling  @river251 

The Ren V claims an even 150 ohms or less over the entire music bandwidth

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. 

Renaissance Mk V Pre Amplifier technical specifications 

@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.

@mulveling

it specifies a recommended load impedance of greater than 300 ohms

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.

600 ohm output transformer termination?

It’s absolutely not needed here. In fact, I think it would be a very bad idea to feed a Ren V into 300 ohms - that’s the lowest impedance that it will work “ok” with. I’ve used the Ren V into 100K ohm tube amps no problem. I now use the VAC Master, same circuit, same specs, same deal (just better sound quality from premium parts and PSU). Also works great into a SS amp. The “> 300 ohms” spec is intended as a flexibility boast, not a requirement or limitation. Many tube preamps will require a minimum load of 10K or higher, which can become a restriction with some SS amps.