The 6F12P is used (by many) in Europe (mainly) in RIAA circuit. You would wonder why? Well, it’s a great Russian frame-pentode which has high-mu and high-gm therefore driving larger currents at low distortion.
Category: RIAA stages
Tuning the system for ETF
This year I entered the shootout competition and will bring my DHT system to the European Triode Festival in France. It’s comprised of the ER801a stage plus the 01a (if extra gain is needed) and the 4P1L PSE output stage
I will have to swap out the amorphous OPT for the Monolith Magnetic ones as the speaker load is 5R.
It’s going to be interesting!
DHT Phono Stage Test
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:
LCR Phono: design notes (Part III)
My previous design wasn’t good for two reasons:
- 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.
LCR Phono: design notes (Part II)
Introduction
This is a continuation of my previous blog post. I will try to share my experience through the design process of this RIAA stage through these individual posts with an attempt to spark some interest in others and in return to get some valuable input from the experience and knowledge of others. Hope this works!
First stage
Here is the initial design version for analysis. I’m working through this step by step and refining the circuit in every iteration. The initial circuit is very simple. The first stage is key. We want to achieve as much amplification as possible from this stage before we hit the LCR network. The choice of the 6S17K-V valve may appear as a surprise to the ones not familiarised with this valve. Here are some notes from Wavebourn around this valve:
LCR Phono: design notes (Part I)
Introduction
A phono stage is probably one of the most challenging circuits to build in audio. Clearly not for beginners, many make the mistake in adventuring in building one. There are several designs which are simple, albeit many are poor ones. In addition to the challenges related to high-gain and very low noise design, audio enthusiasts really overlook the fact that you need to be able to measure and adjust the RIAA curve for a successful phono stage build. This means that you need an IRIAA signal source and also an accurate LCR bridge to adjust the network. I personally built a great IRIAA box and procured several LCR meters including this one as part of my learning journey of phono stages.
RIAA Preamp Power Supply
Introduction
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.
JFET RIAA Phono: Battery Charger
After enjoying the RIAA phono preamp for several weeks with the new battery pack, I decided to build the battery charger just not to be surprised by the lead-acid battery pack running out unexpectedly.
The lead-acid battery pack is formed by a 12V and a 6V 1.3Ah batteries. Between both batteries there is a total of 6 +3 cells = 9 cells. Each cell requires 2.27-2.30V of charge at 15-24C. Therefore the charging voltage should be 20.43-20.7V. Exceeding this voltage will reduce the battery life. The charging circuit has to be constant voltage, not current.
Rod recommended me a simple but very effective design based on the TL431a. The regulator output voltage is set by R1 and R3. 0.1% precision resistors are needed, otherwise a trimpot should be used as R1. R3 should provide at least 2mA for both Q1 and TL431a operation: 
A simple PCB was built in less than 1 hour. You need to place the BD439 in a heatsink as it will get hot when charging the battery:
In a question of 2 hours the battery was charged back fully. I used it for around 20 days so far I think. The initial current is as high as 370mA and drops as the battery charges and the voltage goes up. The residual charged voltage was about 19.4V. This drops quickly after some current starts to flow through the preamp.
JFET RIAA Preamp – battery supply
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!
differential chokes and OSCON caps
Battery pack
Battery pack completed
Measuring noise
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!
RIAA phono stage completed
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.
