Posted 19th April 2013 at 10:24 PM byrjm (RJM Audio Blog)
Updated 20th April 2013 at 05:04 AM byrjm
Changed the collector load on the voltage amplifier stage to a current source (Q3), as per the Marantz SR2285B circuit.
Also increased the resistance of the feedback connection, R6+R8, there seemed to be no obvious advantage in making it much smaller than the typical load (>10k). The compensation capacitor C2 is increased to match, to flatten the HF response.
The circuit can drive light loads to +20 dB. Of course that's not much of a challenge for this general class of circuit.
I'm a bit stumped as to what the next logical step is from here. Seems to me to depend on what you actually want the circuit to do.
Holiday: Bad weather to a point it was near impossible to take a steady photograph due to the gusting winds. Hiding from the winds meant poor vantage points, very much non-idealistic for photographing landscapes.
Digi-key: The order turned up on the Monday morning, roughly 10 minutes before leaving. The PSU boards must have arrived mid week, a neighbour had those...
The parts order was enough components for one power supply and one amp. The PSU is assembled but untested as yet. The toroidal and 10K Alps RK27 are to be ordered from RS Components later today.
PCB issues: C1-C6... this is where trusting the people who generate default libraries for FreePCB falls down and my inexperience shows. Lead diameter on the caps is 28MIL, default part through-hole diameter is 28MIL... Each lead needed a quick file to reduce its width enough to fit the board. No other issues to report.
I don't think I could have made the overall size of the board...
Where was I? Right, back to the workshop and cue the music.
Sorry in advance for my photos. They get the job done, but some of them look pretty bad now. I'd retake them but the amp has long since been returned to it's owner.
Once both channels were running happily with their new parts on the benchtop power supply I turned my limited attention to the chassis. The phenolic insulators sandwiching the RCA jacks in place were starting to crumble so, some shiny new jacks were fitted courtesy of ApexJr.com for 99 cents. The input wiring was done with 22AWG shielded wire.
The pro Hafler models like the P125 and the P230 have a solid aluminum bracket for the ground connections between the main power supply caps. The DH200 and DH220 models only got some tinned wire which isn't very convenient or nice looking. I fashioned up a new bracket with some scrap...
Posted 16th April 2013 at 01:18 AM byrjm (RJM Audio Blog)
Updated 16th April 2013 at 12:23 PM byrjm
Over the last couple of years most of my interest in audio has been with transistors. I've been slowly teaching myself to read and understand the circuits.
Circuits like this one for example. Not hard, but still a bit too complicated for me to understand without the helpful wikipedia markup attached.
Instead I've looked at primarily at the schematics I have for discrete audio preamplfiers, 1970's vintage typically. Based on what I've learnt so far, I've done up a "test mule" in LTSPICE, shown below.
It's not a circuit you should build. It's for pedagogical purposes, though it does actually work reasonably well - in simulation anyway. Its just a simple starting point to observe how the different parts interact under simulation.
Power supply boards are on their way, as is a full Digikey order for this project. However, no LME49722, which are currently out of stock. Due to that and wanting to keep to a 1 x psu & 1 x amp layout, the amp board underwent a quick re-design. It's now duel mono layout, utilising two LME49990.
Ordered LT1033/LT1085 for the PSU, these should give 3A each [typical]. Plus, six 4,700uf caps to help out. The plan is to run the rails at +/-15Vdc, with a 120VA 2x15V toroidal transformer.
Will place an order for the prototype amp boards on Saturday, just before I go on holiday.
Update (13-04-11): The PSU PCB's still haven't arrived, Plus UPS failed to deliver my Digikey order, this is now in limbo as they're saying "A correct street name is needed for delivery" ... I've talked to both Digi-key & UPS (U.S.) and they have the correct full address, UPS (UK) I can't get hold of... my translation is "Delivery driver can't read a map!"...
Posted 9th April 2013 at 06:40 PM byalexcp (My DIY projects)
Updated 12th May 2013 at 03:49 PM byalexcp(Added better photos)
I built this one a while ago but could not find time to post.
This is a Class B power amplifier that follows the Ultra-LD Mk.3 amplifier design published by the Australian Silicon Chip magazine in 2011, which in turn draws heavily from the concept of a Blameless amplifier devised by Douglas Self. I used a different power supply and speaker protection, and changed the grounding scheme vs what was published.
The distortion at 1 kHz is below my measurement capabilities, and the amplifier does sound very nice, although it seems to give more sibilance than my Class A amplifiers.
The acid test I use is the dual CD "The Very Best of Placido Domingo" album published by EMI Classics. Many amplifiers I have heard cannot deal well with Domingo's tenor, esp. on louder and higher tones. Class A amps, particularly those using simple internal structure like the Zen series, perform well in this test. This amplifier seems to do better than any other...
Posted 9th April 2013 at 06:10 PM byalexcp (My DIY projects)
Updated 11th April 2016 at 08:24 PM byalexcp(Added better photos)
Big, hot, and heavy! Just completed it and adjusted for distortion, have not had yet a real chance to listen to it. (Update: I have listened to this amp for some time, and it seems to be a keeper, at least for now.) The discussion is here.
The build is in a 4U/400 case from modushop; each side has two 200mm heatsinks, each holding six MOSFETs (three complementary pairs) and a biasing circuit.
The construction is dual mono (separate transformers for each channel) with CRC filtered +/- 18V rails, quiescent current is 3A per channel. Eight Mundorf MLytic® HC High Current Power Caps and two 300VA toroids occupy the most of the chassis, while the actual electronics is mounted on the sides.
Now I need a balanced preamp. I contemplate building some variant of BoSoZ, but am also thinking about a SuperSymmetric balanced preamp using JFETs or tubes...
UPDATE: No- and low-feedback amplifiers have no (or little) control over output errors...
The amp uses low-voltage 12AE6A tubes for voltage amplification and BUF634 solid state buffers to drive the load. Each tube houses two diodes (unused) and a triode, here loaded with a CCS. The whole thing runs off a 24VDC power brick and consumes maybe 150mA after the tubes heat up.
The DC operating point of the triode can be adjusted with a multiturn trimpot. The original AudioXpress article talks about the influence of the operating point on the sound and suggests to trim for symmetrical clipping, then adjust for the subjectively best sound.
The aha! moment came when I connected the amp to my sound card and looked at the distortion spectrum. On small signals (~300mV output into 300 ohm) the distortion is 2nd harmonic only. The operation point can be trimmed to the point where the the 2nd harmonic goes into the noise floor! I've read about this but...
The exposure 2010S CD player has an easy internal layout, where most of the components sit on a large PCB.
The original clock is based on the 74HC04N chip located in the middle of the PCB, to the left from the three white power connectors connecting the toroidal transformer. A close-up picture shows the crystal resonator wrapped in a black heat-shrink tube, two 33pF capacitors on each side of the resonator (those with black dots on top), and a resistor R75, which also serves in the circuit. The oscillator is built on the single invertor (pins 13 and 12 of the IC).
The original clock is generating 16.9344 MHz at 5V CMOS levels.
To install XO2.2, I removed the resonator and the capacitors, leaving the resistor in place. I connected the signal cable from XO2.2 to the...