If you read the previous article on how quickly we can become deaf through negligence or just plain stupidity (like this scribe) I have managed to get some gems together for the DIY types.
Firstly, for those wanting to repair car amplifiers, Perry Babin has done an incredible job in keeping to the facts and make his tutorials available for free or chargeable as a download. There has been a lot of time and effort going into this so full marks to the author for doing such an amazing job. I guess the best way to get started is from this link: Basic Car Audio Electronics. Continue reading “Car Audio – More on amplifiers and SMPSU”
Using the first article to get our creative juices flowing I found some more well written well designed amplifiers using tubes and semiconductors.
The design link below shows various permutations to feed a high voltage swing with little distortion to an emitter follower output stage. This circuit, titled Hi-End Hybrid Amplifier 100W uses a high end K&K solid state current generator (source) in series with a pentode voltage amplifier. The tube in this case can be a Siemens D3A or Russian 6C45.
OK, so what is our pitch here?
High safety margin, for a 100W amplifier using 4 x MJE15032 and 4 x MJE15033 output transistors in the output, the input to the current amplification circuit is through a capacitor (DC block) from the voltage amplifying pentode and an LM308 op amp for stabilising the DC offset at the output, removing the need to apply manual adjustment.
The MJE15032/MJE15033 can be obtained at a good price from Mantech (Communica advertise only the PNP at R17 each excl).
An auto-bias heater filament Mosfet load headphone amplifier
Quite a mouthful but nice clean design pulls all the stops in hybrid design. Design found here.
Hybrid vacuum tube/solid-state audio power amplifier
Nice design which will break the bank amplifier found here – this amplifier uses an 6AN8, a pentode-triode combo. The output devices are BUZ900 and BUZ905. The 6AN8 may not be that easily available in South Africa but the good news is that Mr. Valve lists this valve for R150.00. Based in Pretoria, excellent service.
The BUZ900 and 905 are expensive, TO3 equivalents include the Hitachi 2SK134 and 2SJ49. Available (no price given) at Mantech otherwise Yebo listed as ECF20N20 and ECF20P20 going for over R300 per pop. Ouch! Note that these are lateral type MOS devices and because of the TO3 can this amplifier is ideal for experimenters wishing to modify the output stage with cheaper alternatives. After all, we are looking at the voltage gain devices, chiefly pentodes driving Mosfets.
Tube and Semiconductor Combos – the best of both worlds
An interesting project on the web is that of a 6SN7 Mosfet audio amplifier capable of delivering 50W per channel. The article, written by Simon Brown is called Amplifier War and Peace – An Hybrid Amp
I am not a big believer in that tube amplifiers are so much better than that of solid state but I do believe a properly designed amplifier using both devices can address problems, both in sound reproduction and ease of build.
What I DO like about tubes is the almost ease at which one can build a pre-amp or power amp by simple point to point wiring. This type of wiring is also notoriously the best way to build these amplifiers – easy to change layout, reduce noise, unnecessary oscillation and to modify at a later stage.
The project, found at Audio Xpress features two 6SN7s in a current source configuration driving a complementary pair of Mosfets, IRFP140 and IRFP9140. All aspects to the design are covered but the author does warn that it’s not for inexperienced builders. Here of course we must also make mention that the V2 is running at 300V on the anode. The power supply is well thought out. The entire project can be built on prototyping board.
Another project which caught my eye and also under Audio Xpress is that written by Stephen Moore – A Hybrid Valve MOSFET SE Amp – Complete Project. With a smidgen of Nelson Pass in the constant current source and gate bias circuitry we, like the above circuit, have two 6SN7s or similar acting as the voltage gain stage to drive the source follower IRFP2907Z. (using the John Broskie Aikido amplifier approach).
Both these articles are very well written with a lot of insight and explanation as to the shortcomings of Mosfet output devices, work arounds and just as important, driving these devices with high voltage swings from tube devices.
The Taga HTA700B V2 SE Hybrid amplifier boasts the following:
Power Output: 2x45W RMS / 4ohm; 2x35W RMS / 6ohm; 2x26W RMS / 8ohm Class A/B
Vacuum Tubes: 2 x 12AX7B
THD: Less or equal 0.1% (at rated power) Signal/Noise ratio: =>88dB
Frequency Response: 20Hz – 28kHz
Inputs: RCA stereo CD, USB (for computer)
Outputs: Stereo RCA with variable volume control
Connectivity Wireless: Bluetooth® v4.0
Bluetooth® Profiles: Advanced Audio Distribution Profile A2DP
Headphone Impedance: 32-320Ω
Headphone Output Power: 1W 160Ω
USB Supported Data: Asynchronous 24bit / 192kHz
True high-speed audio processor CM6631A
DAC Cirrus Logic CS4344 chip
Included Accessories: Bluetooth® antenna, USB cable,
As an addendum to our previous article “Alternatives to Tube Output Transformers” we thought it pertinent to add a bit of local flavour, Mars Amps in Paarl to the mix. No, I have not met the gent but Karel Mars is a well known tube guru in the country and lends his expertise to the DIY community either through workshops or on the local avforums website, found here. (OPT winding).
Mars Amps has a variety of tube pre- and power amplifiers available in kit form, shipping both locally and internationally.
As mentioned in a prior article, Class D amplification is nothing new, modern switching components and topologies paving the way to resilient and high quality amplification circuits.
We can also draw a parallel between modern switched mode power supply design, H-Bridge motor control and that of Class D, all benefiting from newer ultra fast switching transistors, the bug-bear of the first class D and switching power supplies. Continue reading “Class D Chips”
As a young (or old) student it takes some time to get one’s head around the design idiosyncrasies of solid state amplifiers. Truth be told many of us found tube nomenclature and circuit design easier to comprehend than anything that followed “hole” theory. Big bad things come to an end and eventually we found it easier to crib other designs.
There is a lot of theory put into amplifier design and when one looks at these complicated circuits often the protection circuits are more sophisticated than the gain stages. Purists believe the only true amplifier is a piece of wire with gain. Engineers often criticise the protect circuits of creating their own problems, along with causing inferior quality reproduction.
These are the major problems come across in the design of any amplifier:
Heat – is there sufficient cooling for heavy duty work?
Can the output transistors go out of their Safe Operating Area? (known as SOA).
Can the amplifier drive a 2 Ohm load when minimum load spec is 4 Ohm?
The dangers of mains voltage boosting and sagging?
Oscillations and instability.
Over driving and distortion.
Loudspeaker protection – expensive lessons to be learnt.