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Sources
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Paper Manual
Hafler.com Manual
Blue Shadow's ManualHafler.com DH-220 Manual
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Year197919807/19801984
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Document #929201929204929203LM128
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Serial #3952043N/A3102021N/A
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DH-200 Differences
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Component
1979 Manual (929201)1980 Manual (929204)7/1980 Manual (929204)
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R2847Ω 1/2W47Ω Carbon Comp
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R3447Ω 1/2W47Ω Carbon Comp
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R392.2Ω 1/2W.5Ω 1/2W.5Ω 1/2W
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C2
390pF 500 Disc390pF 500V Mica
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C3.001µF 100V Disc.001µF 100V Film
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C4.001µF 100V Disc.001µF 100V Film
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C6270pF 500V Disc270pF 500V Mica
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C7.1µF 100V Disc100µF 80V Electrolytic
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C8.1µF 25V Disc.1µF 100V Film
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C9560pF 500V Disc680pF 500V Mica680pF 500V Mica
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C10.1µF 100V Disc100µF 80V Electrolytic
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C11.01µF 100V Disc.01µF 100V Film
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C12.1µF 100V Disc.1µF 100V Film
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C13390pF 500V Disc390pF 500V Mica
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C1847pF 500V Disc680pF 100V Mica47pf 500V Mica
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C19.005µF 1000V Disc.0047µF 100V Film.0047µF 100V Film
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C20
.01µF 100V Disc.01µF 100V Film
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C21.01µF 100V Disc.01µF 100V Film
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Q1BC546B2N55502N5550
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Q2BC546B2N55502N5550
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Q3BC546B2N55502N5550
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Q4BC556B2N54012N5401
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Q5BC556B2N54012N5401
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Q6BC556B2N54012N5401
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Q9BC546BNP2222NP2222
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Q121A092N34402N3440
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Q131A102N54152N5415
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DH-200 Oddities
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While working on my two DH-200 I've located a few inconsistencies which I can't yet attribute to design revisions. I've cataloged them here.
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1. On my amplifiers C18 was connected between ground and the gate of Q14 instead of between ground and the drains of Q14 & Q15. The pictures of the amplifiers within the manual also appear to have this connection. So is the schematic wrong? Or did someone in the assembly make a mistake for a few years? In the DH-220 manual this inconsistency is resolved and the pictures appear to show C18 connected as shown in the schematic.
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DH-200 vs.DH-220
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The DH-220 appears to be an evolved version of the DH-200. Here I did my best to analyze the topology changes between the two amplifiers. It is by no means a definitive comparison, often I am making speculative oberservations and I do my best to make it clear when I am mostly guessing.
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1. The DH-220 adds an additional potentiometer to adjust the DC offset of the amplifier. This resulted in a redesign of the input network and tails of the two input differential pairs. I believe the offset adjustment works by balancing the tail currents between the two sets of input pairs. By turning the potentiometer one decreases/increases the current in one pair and increases/decreases an equal amount in the second. This adjusts the output voltage of the differential pairs, and thus the entire amplifier.
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2. In the DH-200 the VAS was composed of what I believed to be two unusual arrangements of a darlington pair. Upon inspecting the schematic of the DH-220, I now believe the VAS is composed of a common-collector buffer stage, followed by a common-emitter gain stage. In the DH-220 this stage has been modified so the buffer transistors (Q7, Q10) draw current from the ground rail instead of the VAS output. This may have been to eliminate a form of local negative feedback. I am unsure of the purpose of D9 & D10, perhaps biasing?
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3. The VAS is loaded by a modified version of the Vbe multiplier found in the DH-200. In both amplifiers, the bias current of the amplifier is controlled by adjusting the potentiometer that is part of the Vbe multiplier. This potentiometer adjusts the voltage that is formed across Q9, thus altering the bias of the driver and output stages. In addition the DH-220 adds an emitter resistor to Q9, and an additional resistor to ground. I suspect the additional resistor (R38) sets the overall impedance of the node and thus sets the gain of the VAS stage. This was most likely done to make the gain of the VAS stage less dependent on the device parameters of Q8 & Q11, which may have let to greater reliability or better manufacturing yield.
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5. There are numerous changes to component values between the two amplifiers. As the DH-220 is a higher power amplifier, the gain of the forward amplifier, and the feedback network must have been changed which would explain the numerous changes in component values.
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6. The DH-220 also benefits from extensive bypassing. A number of capacitor were added to the design simply to bypass capacitors already present in the DH-200. C7, C11, C13, C21 & C22 exist definitively as bypass for existing capacitors. Also, at least C17 and C408 were added as additional bypassing.
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7. I've heard the DH-220 had a completely reworked grounding systems. This is difficult to tell from the schematic, except for the replacement of R39 with C23. The original purpose of R39 baffles me. My best guess is the purpose of R39 (and C23) was to create a impedance between the input and output grounds. This would help force the input currents to flow through the chassis and the output currents to flow into the power supply. This is beneficial as the ground noise of the output stage wouldnt interfere with the input stage. The ultimate question this raises for me is why even bother connecting the grounds on the PCB if the goal is to seperate them. The DH-220 effectively does disconnect these stages at ground through the use of C23 which is VERY high impedance at LF.
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