Jul 14, 2018 13:40:13 GMT 12 Owen Y said:

Hi Colin - what was the design purpose of the solitary BC107 transistor at the input of this preamp?

Looks to be a phono stage there and a mic input. You can see the equalisation network and the input selector. On disc the eq network is switched in, on mic the response is flat, and for the other inputs the signal is routed directly to the following all tube stage.

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Thanks garym - the BC107 is just a first gain stage then?

The Mic & Disc inputs go through it, but the Mic looks to be then switched past the EQ stage?

DarkLantern blog - darklanternforowen.wordpress.com

a classic design that still looks great.

performance-wise as good as a modern preamp ?

Jul 14, 2018 14:18:07 GMT 12 Owen Y said:

Thanks garym - the BC107 is just a first gain stage then?

The Mic & Disc inputs go through it, but the Mic looks to be then switched past the EQ stage?

The BC107 and the first (half) tube look to be a low noise amp stage. The BC107 as the input device because its quieter than the tube. The eq is applied for disc, but its not for mic (its left flat). The mic isn't switched past the eq stage as the eq is applied over the whole input stage comprising the BC107 and following tube section. The eq is applied as negative feedback that shapes the response. For the mic that "shape" is flat.

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Jul 14, 2018 17:42:42 GMT 12 garym said:

The BC107 and the first (half) tube look to be a low noise amp stage. The BC107 as the input device because its quieter than the tube. The eq is applied for disc, but its not for mic (its left flat). The mic isn't switched past the eq stage as the eq is applied over the whole input stage comprising the BC107 and following tube section. The eq is applied as negative feedback that shapes the response. For the mic that "shape" is flat.

Ah yes I see the feedback loop RIAA EQ around the BC107-ECC88 (one half twin triode per ch). Thks.

DarkLantern blog - darklanternforowen.wordpress.com

In the era that the SC22 was designed transistors were becoming more common in hifi. When engineers learnt in the 60s that there is a device that exists with low noise, no wear and thus no replacement necessary, and small size they were onto it. So an overall higher performance circuit could be realised in roughly the same space as a four-valve preamp. Four valves were typical in preamps of the time. To have added another valve to make the same SC22 circuit in all-valves would have made the unit bigger, more costly, and more liable to go noisy as the valves wear.
The BC107 is at the input of the MM / microphone stage, right where the advantages of a small, low voltage transistor would work best. It is incorporated within the negative feedback loop with the first valve to make it a hybrid phono stage.
In thinking about this circuit now, I wonder why Radford didn’t direct-couple the BC107 straight to the next stage. It would have made the circuit more stable in the subsonic bass region. But as it stands it represented good forward thinking and is still a good circuit. Although the output impedance of the phono stage is far too high to be able to properly drive the tape-out socket. A buffer, such as a cathode follower, would work wonders here.
Sansui did something very similar in the classic AU-111 valve integrated. The input to the MM stage was much the same configuration as the Radford.
I think circuits like this must have partly inspired Audio Research's William Z Johnson to start examining the hybrid thing in their 90s preamps, like the SP11 and SP9.

AMR-iFi R&D

Jul 14, 2018 14:43:45 GMT 12 michaelw said:

a classic design that still looks great.

performance-wise as good as a modern preamp ?

If it were made today I would modify a few things.
The power supply for one. The capacitor-resistor HT circuit could be made to have much lower noise with later bigger capacitors, and perhaps even use a tube rectifier and choke-input supply. If you were going to town on this circuit you could even make it dual-mono. Regulation if one prefers.
The heater circuit would be DC not AC for much lower hum level.
And a buffer could be included to drive the tape output.
If you’re an anti- tone control purist they could be bypassed.
The input transistor BC107 could be replaced with a modern low noise one like half of a MAT12. (Or LM194 / LM394, Mat02 etc) Perhaps even a low noise jfet instead of a bipolar transistor, such as a BF862 or 2SK170. Then the input impedance would be constant across the audio frequency range making cartridge loading simpler.
And of course nice RCA sockets.
Performance-wise it would be still in the running.

AMR-iFi R&D

Jul 14, 2018 18:33:55 GMT 12 Owen Y said:

Ah yes I see the feedback loop RIAA EQ around the BC107-ECC88 (one half twin triode per ch). Thks.

ECC83 I meant of course, just testing

DarkLantern blog - darklanternforowen.wordpress.com

For 4 or 5 years I ran a Bryston 4BSST with a Croft Vitale pre, a really nice mix as you had the effortless control of the Bryston complimented by the tubes in the pre. I now have Croft's Micro 25 pre and his hybrid Series 7 power which I feel is a better balanced system, I put this down more to the hybrid power amp rather than the pre.

The Croft (UK company) Series 7 looks like a nice simple circuit, built hardwired. IME hardwiring is always good for sound. Looking at the pic there is a 12ax7 input valve, mosfet source follower next to the valve and a class AB mosfet output stage.

AMR-iFi R&D

NAIM Uniti Atom



- “Classic Naim” 40wpc Class A/B amplifier
- Burr-Brown DAC
- 5” LCD screen display (with proximity sensor)
- Wifi antennae (802.11b/g/n/ac-capable)
- aptX Bluetooth
- dedicated headphone amp stage
- Naim's iOS/Android smartphone app
- supports Roon, Spotify Connect, Google Chromecast, Apple AirPlay

darko.audio/2018/07/wheel-love-with-the-naim-uniti-atom/

DarkLantern blog - darklanternforowen.wordpress.com

i remember the uniti's ancestor, the mu-so, getting a rave review from mr knobfeel ...

29 october 2014Naim Mu-so - Knob Feel Review

The quality bar has been raised with the first review for Naim Audio. Although a bit on the large side, this Knob is truly a thing of beauty. I’m 90% convinced the light comes from the angels trapped inside

How are these hybrid?

AMR-iFi R&D

posted in wrong thread ?

Jun 24, 2018 19:55:49 GMT 12 colinf said:

I listened with interest to the interview with Bascom King about his latest hybrid tube-mosfet amp design for PS Audio. In it he explained that using a tube as an input stage instead of SS resulted in a huge increase in believability of the sound. 
I’ve been making hybrid amps for decades and my second amp design when I was 16 was hybrid class A. Bascom uses a 12au7 balanced at medium gain for the input stage and the output amp is SS with a gain of 4. That makes sense to get lower distortion than with a pure tube input stage.
My late friend Peter in Nelson always preferred to use a valve preamp and a SS poweramp as having a valve in the system warms it up a bit over pure SS. And you have the power and control to drive inefficient speakers with a solid state poweramp.
But I sense there has always been a slight resistance from audiophiles to having a hybrid amp like a Counterpoint, or Audio Research's efforts from the 90s. I suspect people generally prefer the design when it is either pure SS or pure valve.
So my question is, is there any love for hybrid amplifiers out there?

I like the sound of valves but I think a fab compromise is the hybrid.  Oops actually I have never heard a hybrid!  What I have is a valve Pre Amp (Ayon) with a SS Power Amp (Pass).  I am happy with that combination.  

I recall a comment by one of the Conrad Johnson team on the SS vs Valve debate.  To them it was a non issue.  They would just use the part that best did the job.  For them valves were best at voltage amplification so that is what they used for that.

Nice amp setup!
The C-J philosophy seems to echo the McLaren car design philosophy. Just use the right material for the job.
If only amp design was like that, it would be great!
There are many design philosophies in cars. Front or rear engine, turbo or normally aspirated, front or rear wheel drive, or all wheel drive.
So too with hybrid amps. Valve input stage and SS output stage (Moscode, Croft etc), or the other way around (Audio Research Classic 60 and it’s brothers, AMC CVT2100 etc)....
Valves do seem to be naturally good at voltage amplification, and transistors good at current amplification. So the valve input, SS output stage configuration would seem to be a good idea.
I suspect though, that a pure valve amp's sound is dominated by its output stage. Swapping the input stage from valve to SS doesn’t seem to diminish that sonic impression too greatly. Hence Audio Research's approach, amongst other manufacturers.
I see Audio Research are back to all-valves in their latest amps, but the power supplies are solid state.
I think that is where SS seems to work best, assisting valves to do their job better. SS could be used in the power regulator, the bias network, as a constant-current device for a triode amplifier stage etc. But the main amplifying device is a valve.
What do you think?

AMR-iFi R&D

I got a copy of the wonderfully crazy book by 'Understanding Tube Electronics' around 1984 by Harvey Rosenberg of Moscode / NYAL / Futtermans fame which influenced me to build a number of Hybrid amps. I published one of these in Audio Express many years ago which was interesting for using a Beta follower to drive the output FETs. This is nice as it adds a current source to the valves to make them linear and give them a bit of grunt. Measured <0.1% distortion at 50W with no loop feedback. Did seem to have some of the good aspects of both types of devices. Happy to pass on the article or circuit diagram to anyone interested in DIY.
Some day in the distant future I'd like to re-visit this with the emergence of the 6H30 valve which seems ideal for this application. (Bigger linear voltage swing than ECC88 / 81)

At the preamp end of the scale, Allen Wright (Kiwi turned Aussie audio guru) has a fair few Fetted preamp designs in his "Preamp Cookbook" Mostly using them in a sort of totem pole combination with the input valve.

Simon Brown
Design Build Listen Ltd,
Makers of The Wand Analogue Products
Distributor of Hana Cartridges

I remember hearing one of Allen's Vacuum State hybrid preamps (RTP4/5?) a few years back & also his DPA differential 300B power amps around the same period.
Not long before Allen's passing.

DarkLantern blog - darklanternforowen.wordpress.com

I’d love to see that article and schematic!
Triodes seem to be capable of very low distortion when used with a constant current source (CCS). The CCS can be solid state or valve. The 6H30 has half the gain of a 6dj8, 15 vs about 30 times. So the poweramp using only one gain stage with no NFB with a 6h30 wouldn’t be very sensitive. Directly heated triodes purposely run on low heater voltage have very low distortion, around an order of magnitude lower than an indirectly heated triode. But most DHTs have very low gain.

AMR-iFi R&D

Schematic as requested, if it is of interest, I can send the article via a PM. (Owen can kick this over to DIY if it is more appropriate there)
The 6h30 is indeed getting marginal for gain but most have way too much gain anyway.
probably you'd use this with a valve preamp....

Simon Brown
Design Build Listen Ltd,
Makers of The Wand Analogue Products
Distributor of Hana Cartridges

Huh! That's virtually the same amp I made back in 1989! Except I used a 2sk147 jfet in the input stage instead of a bipolar transistor.
I fiddled a lot with this circuit and eventually used a 6dj8 and no transistor. I elevated the top triode voltage so that there was around 40v on the 'inbetween' resistor.
6sn7 has about 5db lower distortion than 6dj8 in this circuit and almost enough gain.
I always wanted to make a balanced-bridged version of this amp to get even lower distortion and enough gain.
How did you become interested in that style of input stage?

AMR-iFi R&D

The input stage concept I got from Morgan Jones book "Valve Amplifiers" where he discusses SRPP vs Mu-follower vs Beta follower. (Though he doesn't use diodes for bias).
My earlier amp (about the same time as yours from the sound of it) was a SRPP 6DJ8 / E88CC driver (With 'classic' Hitachi power FETs)

Simon Brown
Design Build Listen Ltd,
Makers of The Wand Analogue Products
Distributor of Hana Cartridges

an interesting look at a modern hybrid...

In this video Kevin compares the customer's interpretation of the sound of this McIntosh hybrid (valve input stage, transistor output stage) to the sound of a Prima Luna pure valve amp with EL34 output valves on bookshelf Focal speakers. The customer claims that the Prima Luna was a night and day difference for the better after he swapped it in, and that the McIntosh hybrid is not a tubey sounding amp. Also that hybrid amps of this type would be better for long listening sessions because of less wear on small valves, and small valves look good on an essentially solid state amp.
That would agree with my experience of the valve input stage, transistor output stage design. It just isn’t “tubey” sounding like the opposite topology, transistor input stage and valve output stage. That type sounds more tubey and has the sonic advantages of a valve output stage coupled with the low noise and zero wear of the SS input stage.

AMR-iFi R&D

Here’s another example of the transistor input stage, valve output stage philosophy. It's the Music Reference RM-200 by Roger Modjeski. It has driver tubes but the input stage is a low noise transistor.

AMR-iFi R&D

Brings back memories, the RM200 were imported here 25-30(?) yrs ago.
I had no idea that it was a hybrid design.

DarkLantern blog - darklanternforowen.wordpress.com

never heard the rm200.

did try the earlier all tube rm9 for a while (with el34 output tubes).

nice sounding but a bit dark and polite with my maggie mg1b.

Maybe I was thinking of the RM9?

I did try Roger's Pre-Pre phono step-up - 6DJ8s of course.
(Now there's maybe a good application for a SS or hybrid design, low noise/hi gain - on paper at least.)

DarkLantern blog - darklanternforowen.wordpress.com

denco brought in the rm9 power amp, rm4 head amp and rm5 preamp.

My father uses an RM-9 with his Maggies. I always thought it sounded lighter and more open with the gain switch on high. Roger wanted his amps to be as reliable as possible but the biggest problem with this amp was that gain switch. The contacts inside corrode over time and it’s a real pain to have to take out the circuit board to change it. Or just leave it on high gain so it is out of the circuit electronically.
The RM-9 was all tube with two 6dj8s on the input stage each channel operating in differential configuration. You needed to adjust the dc balance of the driver stage as it was direct coupled to the input stage. Because the input was a valve, the dc value would drift slightly with wear over time, needing occasional adjustment. In the RM-200 Roger sought to eliminate the drift of the first valve and replaced it with a dual matched transistor which needed no adjustment.

Owen, what did you think of Roger's pre pre RM-4? How would you configure a hybrid pre-pre?
For now my own pre pre uses a 417 valve with a SS highly regulated power supply, technically making it hybrid.

AMR-iFi R&D