Time ago I wrote about this sterling Russian valve. It’s extremely linear in triode mode, sounds superb and isn’t microphonic. My friend Paul LeClerq has used it as first stage of his guitar amplifier and is delighted. A real dormer one. I hope it doesn’t disappear when valve hoarders go out and grab every big lot of valve that exists. Anyhow, I have more than I need myself, so I’m not worried.
High gain stage with DHT
Some time ago a colleague (Shawn Fox) contacted me to find out whether I could test some rare high-mu DHTs. I didn’t have them in my stash, so he offered to send them across for testing. He was quite keen to find out the performance with a gyrator load due to the particular characteristics of the DHT in question. The valve in question is the CX-340. There isn’t much information about this valve am afraid and coincidentally, Thomas Mayer (Vinyl Savor) wrote not long ago a review of this valve.
Tracing the curves, the first step
The high anode resistance as well as the low anode current in which this valve operates makes it a real challenge to implement successfully. Hence, this is why the gyrator load plus an output follower stage comes into play as the best companion for this valve. Before we look into the circuit itself, I submitted the 40 valve to the mercy of my tracer:
- Input capacitance was too high due to Miller effect.
- Overall gain wasn’t enough: 55dB was marginal as 60dB would be ideal for an MC stage. Obviously this doesn’t apply to an MM cartridge where 40dB should be more than ok.
I ran my JFET folded-cascode RIAA preamp for more than a year with batteries. Charging the batteries has been painful enough for me to decide to look for alternative supplies
A DC supply for an MC stage is not an easy task to accomplish. My battery pack ran out to a point that frequent recharging periods became a real nuisance, so decided to build a DC supply.
It’s been some time since I built my JFET shunt-cascode RIAA MC stage. I have listened to several well know records for some time with this phono stage in my system and others. I took it to the London Circle Audio meeting recently and it was very well received by most of the circle members. I even had some requests to build units!
Now I can say that it’s completed. Albeit I haven’t built the charger for the lead-acid battery pack, it sounds fantastic and has great autonomy. Long gone days were I was recharging my NiMH pack! Now I can relax and enjoy music without worrying about charging the batteries now and then. I haven’t measured the voltage across the battery cells so far.
Usually I play my John Coltrane records. Not just because I personally find Coltrane to be one of the best musicians ever existed on earth, I have some specific songs I use for testing. When I was younger, I used to listen for hours the same Coltrane records when learning saxophone. I was keen to listen to every single detail on the phrases, solos and arrangements of the Quartet, Quintet or the Sextet formations. As a sax player, you learn many solo parts, tunes and you know by memory every expression made when blowing the horn. If you don’t have any Trane’s record or even you haven’t heard any of his compositions, I urge you to seek his wide and bast repertoire. From “Love Supreme” to “Giant Steps” and many other gems. Several years ago when I started to rebuild my vinyl collection here in London, I found this excellent compilation of Coltrane: “More lasting than bronze“. It’s a great sample of Coltrane’s music. I use it to test my system. The horn bass notes, the pulsed bass, drums and pianos have a great level of dynamics and detail I constantly listen to. It grabs my mind and takes me to another planet. That is music, more lasting than bronze…
I built a new pack of 12V+6V lead-acid batteries to provide +/- 18V for my JFET RIAA MC phono stage. Despite the bad reputation of these being noisy, Geoff tried them with great success. The Haze brand are the recommended and the low capacity ones (i.e. 1.3Ah) are very quiet.
I decided to build and test it. As the proof is in the pudding!
Here is a simple test on my workbench of the 18V battery set with a 20mA load to simulate the RIAA stage consumption. The LC is formed by a 33mH choke in differential mode + 100uF/20V OSCON capacitor.
No twisted cable pair, just banana alligator clips. The noise level is really low and is obvious that 3.3μV 50Hz hum is picked from the workbench. I also listened to it for a while and can hear the difference. I monitor the FFT with no average and lower FFT size and also couldn’t see any spikes due to chemical reactions. They may happen in future though 🙂
The performance of this battery pack is outstanding, so far so good!
FET RIAA Phono Stage
Finally completed today. It seems impossible from me to get a project finally boxed properly. I’ve done it this time with the RIAA phono stage built recently thanks to the help and guidance from Rod Coleman. This MC stage has a gain of about 70dB for MC cartridges and currently running my DL103a with a 200Ω input load. The beauty of the folded cascode (or shunt cascode as Rod refers it to) is that Miller capacitance is not a problem thanks to the fixed voltage at the drain of the FET input stage. This helps us to have a low input capacitance stage. The second stage is op amp OPA637.
I need to take final measurements of this Phono stage but this is how it looked when I was initially testing it on the bench:
Nice RIAA compliance to +/- 0.1dB. LF noise and gain levels made it difficult to capture the LF response below 100Hz.
The distortion is also really nice with less than 0.025% @ 1kHz and nice harmonic distribution thanks to the CCS at the output which is forcing the operational amplifier to operate in class A:
This stage was well received at the recent London Audio Circle Meeting. It has a nice clean and detail sound. I tried it with multiple MC and even low MM cartridges and has a nice response across the whole bandwidth. Great bass and delicate treble.
I think it is a very quiet phono stage given its high gain. I really like the overall sound and response and I guess that the negative side of it is the 18V battery pack made up of PP3 rechargeable batteries. It needs charge every two weeks and may be a pain in the back. I will try a DC supply, but will have to be really quiet to substitute my battery pack.
I made the following changes to the RIAA stage to improve its accuracy:
- Measured all caps
- Adjusted 75uS network to compensate for the capacitor variance and the cascode output impedance
- Adjusted 318uS and 3180uS networks to compensate for the capacitor variance
All resistors were adjusted with a pair of 0.1% SMD resistors to avoid series connection to reduce noise level.
As shown below, the response is really good matching at less than 1% for frequencies above 50Hz. the error at low frequency is due to the rumble low pass filter, the hum and the soundcard LF response which introduce a slight distortion at very low frequency in the MLS measurement:
|Frequency (Hz)||Test (dB)||RIAA (dB)||Error|
Now is the time to upgrade my current RIAA stage. A bit unorthodox approach for a valve enthusiastic but you will see me now with my hands on sand. A very simple JFET input stage with a shunt cascode pair to drive the passive RIAA network. The final gain stage is an OPA604 with remaining part of the RIAA network. The Op Amp is forced into class A by a nice pair of BC847C forming a CCS.
Given the shunt cascode bipolar is an PNP Darlington formed by 2 BC860C low noise and the CCS has also SMD devices, I started by soldering these into 4 PCB adapters for SOT-23 devices
More to follow…(just a start)