AGC (“Another Gain Clone”) Amplifier
Last updated: January 15, 2014
Used in Henk’s student room
Synq Audio Research X.TRM.1 Turntable
Goldring G800 cartridge
Philips 9710M speakers in Briggs (distributed port) cabinets
My junkbox contains several NOS chip amplifier ICs (TDA2003, TDA2030, TDA7262, TDA7296, LM386 and LM675T). After a prolonged stay in the attic, I thought I should do “something” with them. Mother Maria insisted that a stereo set should be built for son Henk who has left the home and is a student of mathematics at Groningen University.
Initial attempts at building a small stereo amplifier with TDA2030 ic’s were not very successful. The amp worked, but I did not like its sound. Reasons for this may have been: (1) It did not contain an original TDA2030 i.c. from Telefunken or SGS-Thomson, but a copy from the former German Democratic Republic (A2030). (2) Since my circuit was built of recycled parts, some electrolytics may have been leaky, or have shown high ESR. For my second attempt, I used TDA7296 chips and that amp turned out to be a real winner! The TDA7296 is a DMOS i.c. which is (optimistically) rated as a 60W power amplifier by SGS-Thomson. A more realistic estimate of its output is 20 Watts r.m.s. in 8 Ohms. Sufficient for a student room.
The circuit which I employed came right from the datasheet:
The “mute” and “standby” inputs were both connected to the positive supply rail since I did not need mute and standby functions (and, if pins 9 and 10 are connected to ground or left floating, the IC will not produce any sound - as I found out by trial and error!). Pin connection is as follows (the cooling tab is at the back, and the IC is looked at from the front):
On a Polish website, I found a design for a (single channel) PCB which worked well (PCB is shown from the component side). Please note that the design should be scaled before printing (in such a way that the TDA7296 pins fit). And if you’re going to etch the board, you should mirror the drawing with a graphics program to get a view from the soldering side. Also, don’t forget the two wire links during component mounting! And mount the i.c. on a suitable heat sink.
Each channel of my (pardon: Henks) amplifier is fed from a 2 x 20 VAC transformer (salvaged from a damaged Philips stereo set), a heavy bridge rectifier and two 10,000 µF electrolytic capacitors (one for each supply rail). At the amplifier input (left end of C1) I mounted a 47k log volume control. The amp can be driven directly by a CD player or any other line-level audio source (IPOD, tapedeck, etc).
We performed extensive listening tests in our living room, using the Jericho horns as loudspeakers.The amp produces a lively and detailed sound. A direct comparison with a commercial power amplifier (100W/channel in 8 Ohms, 200W/channel in 4 Ohms) did not put the AGC amp to shame. During prolonged listening, the following differences were noted:
1. Low-end control of the commercial amplifier (costing at least 10 times as much) is better than that of the chip amp. In most recordings, this is hardly noticeable but in bass-heavy, demanding passages it can be heard. The chip amp will be appreciated more by lovers of classical music than by rock and heavy metal fans.
2. Small details of recordings are audible with the chip amp, but can be heard even better when the commercial power amplifier is used.
Yet, the chip amp gives a very good account of itself. It beats most affordable stereo sets which we have heard.
Please note that we have used only high-efficiency full-range loudspeakers, not power-hungry speakers with complex crossovers.