Present Phono
Latest Schematic – updated July 2024
Component Types
| 197k input made up of 150k and 47k | – | Vishay S102 |
| 13k8 made up of 12k and 1k8 | – | Takman Rex 50 carbon film |
| 3k18 made up of 3k and 180R | – | Takman Rex 50 carbon film |
| 10k D3a anode | – | Mills MRA12 wirewound |
| u1 | – | Russian FT-3 teflon |
| 100n EQ | – | Russian silver mica 0.4% |
| 34n2 EQ made up of 33n and 1n2 | – | Silver mica 500V 1% and 5% |
| 22u and 150u | – | Black Gate VK |
| 220u+220u | – | Black Gate Wkz |
| Grid Stoppers | – | Allen Bradley CC |
| Screen and PSU decouplers 1k & 47k | – | Kiwame 2W carbon film |
| 10k 2nd stage power | – | Welwyn W24 wirewound |
This is the latest version in a long journey and encompasses what I believe to be the best solution for a phono stage. What follows are my reasons for choosing the elements I have.
Stage 1
This stage sets the scene. I’ve tried nearly all popular combinations from cascode, mu follower, SRPP, hybrid cascode and some others however the winner for great sound was a plain triode, resistive loaded. So why is the stage a pentode? First however, I went from the popular triodes, 12AX7, 6SL7, 6072 etc to d3A because I wanted to try high gm as I had heard that this provides better “jump factor”; more accurate dynamics. It also allows for the EQ to be driven better and so sounds more even handed top to bottom. It’s no coincidence many DIYers end up at this configuration.
The much lower output Z, an order of magnitude lower than say a 12AX7 or a cascode enables a much lower impedance EQ filter and that improves sound quality by quite a margin.
The disadvantage of these types of valve is high input capacitance so no good for a MM input and also it can bring MC cartridge resonances into play when combined with a step-up transformer. A pentode has higher gain and very low input capacitance. At these small signal levels, distortion is not an issue and due to higher gain, noise is not an issue. I think these factors are audible and I prefer the result using a pentode.
Biasing can be resistive in the cathode however that will mean a large decoupling capacitor and I try to avoid capacitors in the signal path as much as possible. A LED in the cathode, a battery in the grid or cathode or a diode in the cathode are other options. After trying them all, I preferred the diode.
Stage 2
Much the same as Stage 1, I ended up with a relatively high gm valve but this time transformer loaded. I have tried all types of loading and coupling in this position; CCS load cap coupled, choke loaded parafeed coupling and so on as can be seen on previous pages.
Have tried all the stage 1 type biasing methods however for this stage, I preferred the grid battery approach. The 5687 is a great match for the d3A and allows a lowish output impedance. It is as simple as it gets and the gain structure suits my system.
Galvanic isolation with transformer coupling means the amplification stages are not susceptible to earth currents and loops. I hear this as a lower noise floor and so finer details are not obscured. Additionally, I have issues with coupling capacitors as they all seem wildly coloured in some way compared to iron and have audible resonances that get in the way. There is only one in the system and that’s after the EQ as in this case, it is a simple solution whereas any other would be more complicated.
Power Supply
Description as Development Phono PSU. Installed a delay relay to the HT as the power supply ramps up faster than the audio circuit can warm up and draw current which can cause an over voltage situation for the power supply capacitors. Although rated to take it, they are unavailable now so don’t want the risk.
Further Changes
I got enthused again to revisit this after seeing a few build threads on the HiFiHaven Forum. The end to end layout was going to become an issue so decided to rebuild with connectors at the back in the same slide in/out case.
A prejudice of mine was parallel components and so had chosen to live with 33nF in the EQ instead of the calculated value of 34.2nF. I had altered the values in the EQ to get it within 0.3dB 20 to 20kHz which I thought was good but that variation was over the whole band. Reworking the EQ was a worthwhile improvement with a much flatter response in the model and slayed a prejudice as well. It models as a 100Hz to 20kHz -0.03dB with a 30Hz 0.1dB and 20Hz 0.3dB down.
I wanted to secure valves for the foreseeable future and sadly, the MV rectifiers had become unobtanium. I had heard good reports about direct heated rectifiers so secured some DW4 and 5R4GY. The DW4 was easy to slot in as it has the same 4V filament. I didn’t think there was much if any difference to the MVs which was a relief.
I increased the current capacity in the HT to 200mA; revised transformers and chokes which would allow me to run things a bit hotter.
After all these years, I felt the battery bias 1st stage should be tested again as I felt it might not be performing as well as it could so tried an LED which was OK, a more relaxed sound to the battery so preferable but then tried the CREE Schottky diode favoured by Ale Moglia. This produced a much hotter bias point which upped the gain a few dB and I liked this very much. Gain is now modeled at about 51dB excluding the step-up transformer.
Also tried the D3a as triode again but preferred the pentode. It has a lot more dynamic jump from the increased gain. I find that as a triode, the sound is more shut in but could see in a hot sounding system, it might be fine; just not for me. Another reason for the pentode is reduced input capacitance. This article by Dave Slagle of Intact Audio is a good read.
Tried a number of output stage bias and coupling configurations for the single ended transformer stage; direct coupling as before with RC decoupled cathode, capacitor coupled with RC decoupled cathode all using the exact same components in different configurations and finally settled on capacitor coupling with grid battery bias which I had a long time ago in previous versions.
As the design is fairly settled these days, it was time to revisit the resistor types to improve things from the ordinary metal films and try some of the “made for audio” carbon types. The results of that are on this Audio Talk Forum thread.
Late 2024 and the phono has new clothes. The insides are similar to previous versions except this time, they slide out the back. Look inspired by old radios and art deco. Teak veneer sleeve and Rimu veneer front over plywood.
