Driving your amp
A typical challenge we may all face is how to drive effectively our amp via a stepped attenuator or an AVC. I have a 4P1L preamp which drives very well my AVC, however, I have now an LME amp which has a wimpy input impedance of less than 7K.
How do we deal with this? A simple line stage which is capable of driving the low impedance of the amp is what we need in this case. Several options are available, however I settled down for a simple cathode follower.
Why? Because I love valves, and I wanted to play around a cathode follower design here.
A heavy load for your preamp or music source may increase distortion and we don’t want that.
I set myself the challenge to design a simple linestage with a minimum number of power supplies. I could have gone for a MOSFET follower, but hey: I wanted some hollow state stuff in there! Ok, if we look into a cathode follower as the core design, this means that we need at least an HT supply and a filament supply. If we could leverage a bucket converter, we could provide the HT from a LT transformer, probably best to look into two windings to separate the filament supply from an HT one. There are cheap ready build step-up converters for peanuts, and this is what tempted me to explore this solution.
I tested recently some step-down bucket converters and was encouraged by the noise levels and the FR.
The first design, getting us started
Well, a cathode follower is not rocket science. Still, there are some subtle aspect of the follower to bear in mind during our design.
A high-mu valve can help us in maximising the performance of the follower. Yet, we want a high transconductance one as well to keep the output impedance low. Remember that the output impedance of the follower can be approximated to 1/gm.
Not many high-mu valves have high-gm, and we typically settle for high-gm/mu triode-strapped pentodes (e.g. D3a, E280F, 6J51/52P, etc.) or triodes like 6C45P.
I will play here with 6C45P or 6J52P simply because I have many around, and I have created some Spice models which are quite accurate.
I picked up the 6J52P first and looked at the curves to chose a good operating point. Not too much anode current but also away from potential grid current which will introduce distortion. I settled for Vak =125-130V and Vgk=-1.45V to target 17 – 18mA of anode current. This could be done with a 150V HT supply. Probably a cascoded DN2540 CCS feeding a VR to improve PSSR, or Morgan Jones’ statistical regulator.
To avoid a negative supply for the tail CCS, I looked into the self-biased follower. This allow us to have a tail CCS referenced to ground and biasing the valve with a bootstrapped resistor (R2) as shown below:
The tail CCS is not rocket science. A pair of NPN transistors will help improving the output impedance of the CCS. Probably Q1 could be downgraded to a BC550C and benefit its higher hFE.
As we are forced to have an output capacitor (ouch) why not bother to have an input one, albeit small. C1 can be kept to very low value thanks to the bootstrapped R2 which is effectively in parallel with R1 which provides a path to ground for C1.
R3 helps us setting the operating point of the valve. The CCS current is set by R5 and the LED array current is set by R4.
C3 unfortunately is driven by the low value of the output load. With the 7K load we need nearly 5uF to get the LF corner below 5Hz. An oil cap will do nicely here.
Distortion simulations are encouraging. THD is less than 0.007% for output signal below 2Vrms, which is more than I need!
This needs refinement, of course.
Time to play a record and drink some wine. Bye for now 🙂
Hi Ale,
Why not go all the way and do a full SLCF?
Constant voltage and constant current.
Gotta be good….doesn’t it?
best regards
tim
Hi Tim, what is SLCF?
Hi Ale
Super Linear Cathode Follower was the name the late Allen Wright gave to a CF working with both constant voltage and constant current.
best regards
tim
Doh, I should have thought about that one! Thanks for reminding me of its existence! I have played around with an alternative design around this SLCF and you’re right, could be a fantastic addition!
http://www.bartola.co.uk/valves/2015/08/22/a-not-that-simple-line-stage/
cheers
Ale
Hi Ale,
cathode followers also work with output transformers. I just had a (very minimal) exchange about this with Dave Slagle and he thinks this is the snartest way to do a cathode follower.
Best,
Daniel
Hi Daniel,
Sure, but adds a transformer. Broskie covers pretty well the CF with TX on his blog, I suggest you read it. Given cathode current going through the TX implies a high-quality SE-like TX is needed.
I have already too much iron on my system 🙂
cheers
ALe
Love your blog and know you like doing your own thing, however I’ve been playing with the MJ Statistical Regulator idea for a while, with good results – bit of modelling of the true zener behavior as well – documented this over on Audio-Talk where I have some folk currently testing some MJSR boards I made up for this:
http://www.audio-talk.co.uk/phpBB3/viewtopic.php?f=11&t=6204&start=15
Hi Nick, great stuff, it’s on my list to test MJ’s regulator, not far off from the CCS+VR approach I’ve been using for year. Would be keen to test it soon.
Great job
Cheers
Ale
Hi,
Great article. May I have a question?
I would like to use the a triode connected 6Z52P as cathode follower to drive a SV811-10 into class A2 (zero Volts grid bias).
My PSU is +/-120V. What operating point (cathode resistor) would you recommend fro the 6Z52P in this case?
Hi,
The calculation of a cathode resistor is straight forward. Don’t take this in the wrong way, but my advice is that if you don’t know how to do this with the information already presented, don’t even attempt to design a class A2 amp until you learn and experience the basics. Class A2 is not as simple as it may look.
Ale
I certainly have a lot of building experience following tested and working schematics but never went too deep into theory and calculations and I don’t even intend to go there.
I couldn’t find a ready to go schematic for a triode connected 6J52P CF (which it seems will produce a very low output impedance) and since you obviously have experience with those I just wanted to ask for your advice.
Thanks for your reply anyway.
ok. You can use the same bias point as in this SLCF. I also posted the triode curves, so you have everything you need to calculate it yourself. Good luck!
I can point two things here:
1. I haven’t convinced there is a need of the CCS. As far as there is a simulation I’d like to see the graphical advantages of this approach.
2. In the development of the schematic you used a circuit with DN2540. Taking into account the easy way to build a DN2540 enabled CCS (self-biasing even in the cascoded version) why not simplify the circuit?
The CCS provides a much higher output impedance than a simple resistor to the valve, hence the output impedance achieves closer to 1/gm.
You can build the CCS of your choice. For best frequency response, the proposed circuit is much better than the DN2540. DN2540’s transconductance is worse than the NPN option and is also dependent on drain current.
Morgan Jones has written extensively around this topic, so won’t duplicate what is already there.
Cheers,
Ale