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Dynaco SCA-35 - integrated amplifier

The meaning of "integration"

[Dynaco SCA-35]
[Italian version]

Product: SCA-35 Integrated Amplifier
Manufacturer: Dynaco - USA
Price: ~$300 (used)
Reviewer: Harvey A. Kader
Reviewed: January 2001

The term "integrated amplifier" always seems to be a source of confusion for some audiophiles. Just what is an integrated amplifier? Is it just an amplifier like any other, or is it something more? Webster's Ninth New Collegiate Dictionary defines integrate as "to form, coordinate, or blend into a functioning or unified whole: unite".

The SCA-35 was Dynaco's very first integrated amplifier. An integrated amplifier is a combination of a pre-amplifier and an amplifier. So despite its name, it is not simply an amplifier - it is a preamp and an amp. Tube amplifiers require a pre-amp between the signal source (record player, CD player, and tuner) and the amp. The function of the preamp is to amplify the very weak signals from the source to a level which the amplifier can work on to further amplify the signal to room filling sound levels. The preamp contains the volume control, and can also house tone controls (bass and treble). It can also provide a balance control.

The Dynaco SCA-35 is often confused with the Dynaco ST-35. The latter was Dynaco's baby amplifier (see review of the ST-35 on TNT-Audio). Yet there are many features of the ST-35 which are found in the SCA-35. In fact the amplifier section of the SCA-35 is virtually identical to the ST-35 amplifier in some ways, and completely different than the ST-35 in other ways.

The preamp section, though, of the SCA-35 is completely different than the popular Dynaco PAS3 preamp. The preamp section of the SCA-35 is a simplified preamp, and uses different circuitry. This is where the SCA-35 is simplified. A 1964 Dynaco brochure describes the SCA-35 as "…leanly designed, yet there is nothing skimpy about its electrical performance, general features, or even appearance".

[Dynaco SCA-35 front panel]

The front panel of the SCA-35 is gold anodized aluminum. This front panel matches that of the PAS3 preamp, and the FM-3 tuner. The SCA-35 is a compact 13.5 in (L) X 4.25 in (H) X 10.0 in (D). Weight is heavy at 20 pounds, mostly due the heavy output transformers. The cover is brown in color (same as Dynaco ST-70 and ST-35).

The front panel has an input selector (tape head, phono, radio, tape, and spare), a volume control, a channel balance control, and a bass and treble control. At the bottom of the panel are switches for stereo/mono, loudness contour, filter, and AC power. The tape head position follows NAB equalization (for those still using magnetic tapes this is a boon). The phono position follows RIAA equalization.

Speaker taps at the back are for 8 ohms and 16 ohms. Rare hum controls are provided for each channel. The hum controls can be set at the lowest hum level for low level inputs (for example phono). This position gives the highest S/N ratio which is perfect for low level inputs. For high level inputs the hum controls can be set a higher hum levels.

Dynaco has always been famous for elegant yet simple circuit designs. The elegance comes from achieving sonic excellence with minimal amount of electronic parts. Simple design is accomplished by limiting the distance from input to output to the shortest one possible. Dynaco designs are sheer genius.
You have to hear them and see the equipment to believe it. Hearing and seeing are believing!

Output power is 17.5 watts per channel (same as Dynaco ST-35). Frequency response is +- 0.25 db 20 Hz-20kHz. Harmonic distortion is less than 1% from 20Hz-20kHz within 1 db of 17.5 watts per channel. Intermodulation distortion is 0.1% at 1 watt, rising to less than 1% at 17.5 watts per channel.

Unlike the PAS3 which has tone controls (bass and treble) for each channel separately, the tone controls on the SCA-35 are bridged so that both channels are adjusted simultaneously. This is what most preamps do. The design of the PAS3 is unusual for a preamp, and somewhat of a luxury. So the bridged tone controls of the SCA-35 are par for the course.

[Dynaco SCA-35 inside]

The preamp circuitry of the SCA-35 is different than the more complex PAS3. So the SCA-35 is not simply a marriage of a PAS3 preamp with a ST-35 amp. The preamp is very simple. It uses only one 12AX7 tube for each channel, and these same tubes are used for all inputs. The preamp circuitry is on one printed circuit board for both channels together. In sharp contrast the PAS3 uses two 12AX7 tubes for each channel. One circuit board contains one 12AX7 tube for all inputs for each channel, and another circuit board containing two 12AX7 tubes for further preamplification.

The power supply uses two silicon diodes to power both the preamp section and the amp section. A single power transformer (PA774) supplies the entire integrated amp. The is the same transformer as in the ST-35. Interestingly, although the same silicon diodes and power transformer are used for both preamp and amp, a separate electrolytic filter capacitor is used for each section.

The amplifier section is virtually identical to the ST-35 in some ways. Two 6BQ5/EL84 tubes are used in each channel. Just like the ST-35, each channel is on a separate circuit board. However, unlike the ST-35 where the output boards are on opposite sides of the chassis, in the SCA-35 the output boards are right next to each other. The SCA-35 output transfomers (Z 565) are the same as in the ST-35.

On the other hand, the SCA-35 amplifier is completely different than the ST-35 in other ways. In the SCA-35 one 7199 tube is used in the driver section for each channel. Contrast that to the ST-35 where one 12DW7/7247 tube is used as the driver tube for each channel. So the driver tube topology is completely different in the SCA-35 and ST-35.
The 7199 tube and the 12DW7/7247 tubes are different. The 12DW7/7247 is a dual triode tube that has dissimilar sections. One part is half of a 12AX7, and the other part is half of a 12AU7. Interestingly, the ST-70 driver tube is a 7199. So in essence, the tube topology of the SCA-35 is a marriage of the ST-35 output tubes (6BQ5/EL84) with the driver tubes (7199) of the ST-70. Why it is designed this way is not known.
To achieve the same 17.5 watts per channel and similar distortion levels, the designers could have just as well used the total design of the ST-35. Something made them switch to the 7199 tube of the ST-70. It could have been the simpler design of the preamp section of the SCA-35 which led to a switch in driver tube topology from that of the ST-35.

So how does this integrated amplifier sound? Listening tests were conducted with a variety of music (records and CDs). The turntable was a Dual CS 505-3 belt drive. The CD player was a Parasound C/DX-88. Speakers were Paradigm 11se Mark II. These speakers have a nominal impedance of 6 ohms and a minimum impedance of 4 ohms. Room sensitivity is 92dB. Anechoic sensitivity is 89dB. The SCA-35 was able to handle the lower speaker impedance at the 8 ohm output tap of the integrated amp.

It sounds very good for its size and simple preamp design. It is perfect as a bookshelf unit driving small bookshelf speakers. Like the ST-35, it can drive larger speakers in a living room, but only if the speakers are high sensitivity. The SCA-35 does not sound quite as good as a PAS3 preamp coupled with a ST-35 amp. It is thiner in bass response compared to a PAS3 with a ST-35, although a ST-35 is thinner in bass response to a ST-70. The difference is that the SCA-35 is even thinner in bass response than a PAS3/ST-35 or a PAS3/ST-70. Overall, the SCA-35 is a wonderful integrated amplifier and can be listened to for hours and hours.

Market value for a used SCA-35 in excellent to mint condition is US$300.00. Price new in 1964 for the kit version of the SCA-35 was US$104.95, which also happened to be the same price for the kit version of the ST-70 in the same year. A PAS3 and a ST-35 both in kit form would have cost US$136.40. So it can be seen that just in terms of cost, the SCA-35 was "leanly designed". But not "leanly" in sound! Integration has its advantageous.

Copyright 2001 Harvey A. Kader - www.tnt-audio.com

HTML by: David Lundin

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