Uploaded July 2014 | Updated September 2026, 18 hours ago
Description: Whenever I begin an amplifier building project, I always start on a solder-less breadboard. This is the S-Dec S DEC, from the 1970s. The amplifier consists of an OC81 matched pair, and OC81D, and a WN5444: pretty much an OC45 with a different label attached, used as a pre-amp. For more info on this, please see my other youtube uploads. The older style transformer coupling is used in this design. This method of inter-stage coupling results in a higher gain because the transformer ratios can be finely adjusted by the manufacturer; where the transistor collector impedance differs from the transistor base impedance of the following stage, thus preventing matching losses. The main purpose however, of using an inter-stage transformer and an output one is to firstly, split the phases (driver) and re-join the phases in the (output) transformer. Unfortunately, using two transformers in this configuration induces unwanted phase errors in addition to the ones desired. Amplifiers using both output and driver transformers aren't considered HiFi. They're only used when we require a lo-fi reproduction in a system where supply voltage is low: typically 9 - 12 volts, in a portable or car environment for example. Trying to employ negative feedback in these installations is almost impossible. Very often, shunting capacitors will be seen connected around these transformers to prevent unwanted high frequency oscillations. This also limits the HF response. Transistor radios in the 1960s used similar arrangements and one problem was that the radio would only sound ok when the battery was completely fresh. They'd start producing intolerable distortion with a battery that was operating at about half its longevity. The issue was sorted by the mid 70s when the designers used a semiconductor device, used here, as a bias stabiliser, instead of the troublesome output stage resistance voltage divider transistor biasing.
What's wrong with:
spheresound.com/productdetail.cfm?productid=198
Well, apart from the price, over $200.00 the supply voltage will have to be absolute and undeviating to within about 0.1%, okay for a mains supply but you couldn't realistically use it on a battery. The whole point of a circuit of this sort is that it can work from a battery supply, other wise we would prefer to use a 'class A' output stage, consisting of a single output transistor. The design above will easily deliver 1 watt if a bigger centre tapped output transformer was used and heat sink clips were to be fitted onto the output transistor pair.
Description: Whenever I begin an amplifier building project, I always start on a solder-less breadboard. This is the S-Dec S DEC, from the 1970s. The amplifier consists of an OC81 matched pair, and OC81D, and a WN5444: pretty much an OC45 with a different label attached, used as a pre-amp. For more info on this, please see my other youtube uploads. The older style transformer coupling is used in this design. This method of inter-stage coupling results in a higher gain because the transformer ratios can be finely adjusted by the manufacturer; where the transistor collector impedance differs from the transistor base impedance of the following stage, thus preventing matching losses. The main purpose however, of using an inter-stage transformer and an output one is to firstly, split the phases (driver) and re-join the phases in the (output) transformer. Unfortunately, using two transformers in this configuration induces unwanted phase errors in addition to the ones desired. Amplifiers using both output and driver transformers aren't considered HiFi. They're only used when we require a lo-fi reproduction in a system where supply voltage is low: typically 9 - 12 volts, in a portable or car environment for example. Trying to employ negative feedback in these installations is almost impossible. Very often, shunting capacitors will be seen connected around these transformers to prevent unwanted high frequency oscillations. This also limits the HF response. Transistor radios in the 1960s used similar arrangements and one problem was that the radio would only sound ok when the battery was completely fresh. They'd start producing intolerable distortion with a battery that was operating at about half its longevity. The issue was sorted by the mid 70s when the designers used a semiconductor device, used here, as a bias stabiliser, instead of the troublesome output stage resistance voltage divider transistor biasing.
What's wrong with:
spheresound.com/productdetail.cfm?productid=198
Well, apart from the price, over $200.00 the supply voltage will have to be absolute and undeviating to within about 0.1%, okay for a mains supply but you couldn't realistically use it on a battery. The whole point of a circuit of this sort is that it can work from a battery supply, other wise we would prefer to use a 'class A' output stage, consisting of a single output transistor. The design above will easily deliver 1 watt if a bigger centre tapped output transformer was used and heat sink clips were to be fitted onto the output transistor pair.
