valve amplifiers and high efficiency speakers

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05/21/2017 09:53 AM
Electro-Voice EV 209-8A - 8" Full-range

During a visit to Sakuma's Direct Heating website, I encountered a cool SE 801 amp schematic designed for either a Lowther PM6 or an Electro-Voice 209-8A. I googled the EV 209-8A and nothing came up in Audio/DIY centric English language websites. But there were a lot of US dealers stocking the driver from as low as $18 each.

basic specs

After downloading and studying the detailed specifications at the EV website, I chose the cheapest internet seller and phoned-in an order for a pair. 

It cost $40/pair from Full Compass delivered within a couple of days to my doorstep. In spite of their great customer service, their packing left a lot to be desired - see the slightly wrinkled whizzer in the above right picture - the drivers were free floating in the box, not even tied down face to face ;( 

Due to lack of resources to build the recommended 1.8 cubic foot sealed cabinet, I loaded the EV 209-8A in a Heathkit 1.2 cubic foot ported cabinet, which used to house a Jensen P8RL + RP103. 

Heathkit 1.2cf + EV209-8A

Sharp fall off below ~ 90hz, otherwise the in-room RTA result was satisfactory. The tonal balance reminded me of a good mini-monitor speaker with no upper midrange nasties. It was also quite extended in the high frequency and at no time did I crave for a super tweeter.

Further internet research yielded this Japanese website which did an in-depth study of the EV 209-8A. Based on Chrome browser translation, transmission line loading was used to get around the bass deficiency.

Regardless of Qts values ;) and in keeping with JE Labs tradition, a trial in an open baffle is de rigueur. Since my original pair of OBs are currently in storage, I improvised a 55" wide x 32" high flat baffle using a 24" x 24" x 1/2' thick pine plywood baffle from my experiments in the 90s as the core, supplemented by cardboard extension wings.

OB + EV209-8A

Despite the even sharper fall off in mid bass response as shown in the RTA, subjectively, the bass extension was similar to the EV209-8a/Heathkit box combo. But the scale of the sound field produced by the EV209-8A/OB combo was a lot larger than when it was loaded in the Heathkit cabinet.

Since I discovered that my very first high efficiency speaker (a green KS 14703/Altec 755C) was carefully stashed in a box in the basement, I installed it in the OB to take a trip down memory lane. 

OB + Altec 755C

I've covered the Altec 755C in the Sound Practices No. 17 Homebrewer feature many moons ago, so I will spare adjectives to describe the subjective performance of the KS14703/755C. Suffice to say that the above RTA graph shows it all. It is just in a different class sonically and affordability! Perhaps not a very fair comparison for the EV209-8A because this driver with an intact cone and voice coil is worth about 20 times more when they come up for sale.

So even if I couldn't coax Altec 755C performance from the EV209-8A, its street price, efficiency, relatively flat impedance curve and midrange to upper frequency performance quality impressed me enough to bring it to the attention of the Audio/DIY world. 

05/01/2017 11:59 AM
Preamplifier with nice tennis ball tubes..

Had to convert the R101 preamp due to some residual hum which I could not get rid of without changing the transformer topology.. No worries though, plenty of possibilities here. So rebuilt it using some nice tennis ball shaped Psvane 6SN7's. Kinda neat use of a 6AL5 double diode per channel as cathode bias for the 6SN7 whilst operating in SRPP with a line-output transformer. Bandwith from 8hz up to 210Khz.. And no R or C in the signal path, except from the coupling capacitor..
6SN7 and 6AL5 preamplifier
6SN7 and 6AL5 preamplifier
6SN7 6AL5 preamp